What are the theorems in mathematics which can be proved using completely different ideas? ...

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What are the theorems in mathematics which can be proved using completely different ideas?



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I would like to know about theorems which can give different proofs using completely different techniques.




Motivation:

When I read from the book Proof from the Book, I saw there were many many proof for the same theorem using completely different fields of mathematics.

(There were Six proofs of the infinity of primes based on different ideas even using topology.)




I wonder How mathematicians give this kind of proofs.

If you know such theorems and proofs please share them with me. Thank you.










share|cite|improve this question











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  • $begingroup$
    when are two proofs the same
    $endgroup$
    – Bumblebee
    May 7 '15 at 6:38


















42












$begingroup$


I would like to know about theorems which can give different proofs using completely different techniques.




Motivation:

When I read from the book Proof from the Book, I saw there were many many proof for the same theorem using completely different fields of mathematics.

(There were Six proofs of the infinity of primes based on different ideas even using topology.)




I wonder How mathematicians give this kind of proofs.

If you know such theorems and proofs please share them with me. Thank you.










share|cite|improve this question











$endgroup$












  • $begingroup$
    when are two proofs the same
    $endgroup$
    – Bumblebee
    May 7 '15 at 6:38
















42












42








42


19



$begingroup$


I would like to know about theorems which can give different proofs using completely different techniques.




Motivation:

When I read from the book Proof from the Book, I saw there were many many proof for the same theorem using completely different fields of mathematics.

(There were Six proofs of the infinity of primes based on different ideas even using topology.)




I wonder How mathematicians give this kind of proofs.

If you know such theorems and proofs please share them with me. Thank you.










share|cite|improve this question











$endgroup$




I would like to know about theorems which can give different proofs using completely different techniques.




Motivation:

When I read from the book Proof from the Book, I saw there were many many proof for the same theorem using completely different fields of mathematics.

(There were Six proofs of the infinity of primes based on different ideas even using topology.)




I wonder How mathematicians give this kind of proofs.

If you know such theorems and proofs please share them with me. Thank you.







reference-request big-list alternative-proof






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Aug 10 '15 at 10:49







Bumblebee

















asked Nov 7 '14 at 1:34









BumblebeeBumblebee

9,74912551




9,74912551












  • $begingroup$
    when are two proofs the same
    $endgroup$
    – Bumblebee
    May 7 '15 at 6:38




















  • $begingroup$
    when are two proofs the same
    $endgroup$
    – Bumblebee
    May 7 '15 at 6:38


















$begingroup$
when are two proofs the same
$endgroup$
– Bumblebee
May 7 '15 at 6:38






$begingroup$
when are two proofs the same
$endgroup$
– Bumblebee
May 7 '15 at 6:38












13 Answers
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active

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One example of a theorem with multiple proofs is the Fundamental Theorem of Algebra (All polynomials in $mathbb{C}[x]$ have the "right number" of roots). One way to prove this is build up enough complex analysis to prove that every bounded entire function is constant. Another way is to build up algebraic topology and use facts about maps from balls and circles into the punctured plane. Both of these techniques are used specifically to show one such root exists (and once this is proved the rest of the proof is easy).



I think there are other possible proofs of the theorem but these are two I have seen.






share|cite|improve this answer









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  • 2




    $begingroup$
    See more proofs in the book The Fundamental Theorem of Algebra by Fine and Rosenberger.
    $endgroup$
    – lhf
    Jan 26 '15 at 10:33






  • 1




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    You can prove the fundamental theorem of algebra using Lie theory: if $L / Bbb C$ were a finite extension, the complex projective space $PL$ would inherit the structure of a compact commutative Lie group from the multiplication on $L$. But compact commutative Lie groups are tori, and complex projective space in any positive dimension is not a torus. (Credit goes to Mariano Suárez-Alvarez; I learned this proof from a comment he posted somewhere.) You can also prove the FTA entirely algebraically, though I don't remember the details.
    $endgroup$
    – user98602
    May 6 '15 at 3:29








  • 3




    $begingroup$
    See mathoverflow.net/questions/10535/… where 44 answers have been posted.
    $endgroup$
    – Gerry Myerson
    Jul 15 '15 at 7:20






  • 1




    $begingroup$
    There is an algebraic proof that makes use of the Sylow theorems and basic Galois theory.
    $endgroup$
    – Slade
    Jul 15 '15 at 7:52



















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The Brouwer fixed-point theorem. The Wikipedia lists the following methods:




  • Homology

  • Stokes's theorem

  • Combinatorial (Sperner's lemma)

  • Reducing to the smooth case and using Sard's theorem

  • Reducing to the smooth case and using the COV theorem

  • Lefschetz fixed-point theorem

  • Using Hex






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    What is COV theorem?
    $endgroup$
    – Mizar
    Jan 28 '15 at 9:10










  • $begingroup$
    @Mizar, change of variables: en.wikipedia.org/wiki/….
    $endgroup$
    – Martín-Blas Pérez Pinilla
    Jan 28 '15 at 9:14












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    While we're at it, there are two proofs using the Brouwer degree for continuous maps of open sets in $mathbb{R}^n$. One is essentially the same as Hirsch's theorem based on the impossibility of a retraction onto $S^{n-1}$, and the other constructs a homotopy between the identity and identity minus the continuous map.
    $endgroup$
    – Gyu Eun Lee
    Mar 16 '15 at 7:14



















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A large rectangle is tiled with smaller rectangles. Each of the smaller rectangles has at least one integer side. Must the large rectangle have at least one integer side?



What if you replace 'integer' with 'rational' or 'algebraic'?



Here are fourteen proofs.






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    Thanks, this is very nice!
    $endgroup$
    – joriki
    Jul 27 '15 at 17:29



















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As a kind of extreme example, I've heard of this book, reputed to have given over 360 proofs of the Pythagorean theorem. I'm not sure how pairwise different they are, but there's bound to be a lot of variety.



Another one that sticks in my mind is the proof of the Cayley-Hamilton theorem
for real (or complex) matrices. There is an algebraic proof that works for any field at all, but for the reals and complexes, you can argue topologically that the matrix you are working with is the limit of a sequence of diagonalizable matrices, and since the approximations satisfy their characteristic polynomial, so does the limit.






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  • 3




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    +1, and interestingly, the geometric proof of Cayley-Hamilton even carries over to arbitrary fields when one uses the Zariski topology.
    $endgroup$
    – Hanno
    Jan 18 '15 at 13:15






  • 1




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    I'm currently this this, which gives a number of different proofs of the Pythagorean Theorem. Some proofs are very similar to each other, but others are quite different.
    $endgroup$
    – Randy E
    Jan 18 '15 at 13:16





















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  1. $20$ different proofs 0f the Euler Formula
    $$color{Red}{V-E+F=2.}$$


  2. $10$ different proofs 0f the Gaussian Integral
    $$color{Green}{int_{mathbb{R}}e^{-x^2},mathrm{d}x=sqrtpi.}$$







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    This and this also provides some methods of evaluating Gaussian integral.
    $endgroup$
    – Bumblebee
    Jul 15 '15 at 8:17



















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A famous example is the law of quadratic reciprocity. Wikipedia says that
several hundred proofs of the law of quadratic reciprocity have been found.
Many details concerning proofs by different methods can be found at the question at MO.






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    The first example that comes to my mind is the Cauchy-Schwarz inequality.



    Here there is a paper I found some time ago in which the authors claim (I write claim, because I did not check each proof) to be able to show twelve different proofs of the result.



    Actually, there is an entire book on inequalities that starts from this very basic one, with various proofs of it.






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      There are several results of this kind: For example the period three implies chaos theorem (http://mathworld.wolfram.com/PeriodThreeTheorem.html) was proved by Li and York (in 1975) by using the intermediate values theorem for continuous functions in despite the result belongs to discrete dynamics. A generalization of this result (http://mathworld.wolfram.com/SharkovskysTheorem.html) was made before (in 1965) by A.N. Sharkovsky by using a very complicated and different method.



      Also, I note that the most know theorems of L. Riemann were proved by introducing new ideas based on complex analysis. Those of Euler are based on the series...etc.






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        Irving Adler in 'a new look at geometry' gives a circular chain of proofs of alternate versions of the fifth postulate (that if a line striking each of two lines at the same angle, then the two lines do not cross at any distance).



        The idea of such a proof shows that all of these propositions are identical in function.






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          Proofs that $sqrt{2}$
          is irrational.



          I know (at least) four,
          and there are lists of many more.
          I could go on,
          but would be a bore,
          so I'll let someone else
          have the floor.






          share|cite|improve this answer









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          • 1




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            Do a Google search for "proof square root of 2 is irrational". One link, which has a number of proofs, is here: en.wikipedia.org/wiki/Square_root_of_2
            $endgroup$
            – marty cohen
            May 22 '15 at 14:56










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            I found your this post. Thank you.
            $endgroup$
            – Bumblebee
            Jun 22 '15 at 7:43






          • 1




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            Another link with may proofs is cut-the-knot.org/proofs/sq_root.shtml
            $endgroup$
            – Gerry Myerson
            Jul 21 '15 at 6:56






          • 1




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            When writing your answer, did you mean to rhyme each time [: )]?
            $endgroup$
            – Antonio DJC
            Jun 11 '17 at 3:33










          • $begingroup$
            One thing is certain: my rhyming is rarely inadverten t.
            $endgroup$
            – marty cohen
            Jun 11 '17 at 4:49



















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          Robin Chapman gives 14 proofs of $$sum_{n=1}^{infty}{1over n^2}={pi^2over6}$$ here.






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          • $begingroup$
            This article provides a beautiful non-proof (Euler's proof) of this identity. Thank you.
            $endgroup$
            – Bumblebee
            Jul 17 '15 at 8:50





















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          For $ngeq kinmathbb{N}$, you can prove that $dfrac{n!}{k!cdot(n-k)!}$ is an integer from a combinatorial/counting argument. Establish that the formula gives the number of ways to choose a subset of $k$ from a set of $n$, and that automatically makes it an integer.



          Or you can prove that $dfrac{n!}{k!cdot(n-k)!}$ is an integer by showing that the highest power of any prime $p$ dividing the denominator is less than the highest power of $p$ dividing the numerator.



          Several proofs here.






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          • 3




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            You can also prove this using group actions: $S_n$ acts on the set ${1, 2, dotsc, n}$, and the stabilizer of any $k$-set from that set is $S_ktimes S_{n-k}$. So by the orbit-stabilizer theorem, the number of such $k$-sets is the number of orbits, which is $frac{|S_n|}{|S_k||S_{n-k}|} = frac{n!}{k!(n-k)!}$.
            $endgroup$
            – rogerl
            Jul 27 '15 at 17:27



















          1












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          Other examples of such many-proof theorems include:



          1) Harmonic series diverges



          There are at least 8 proofs of this fact based on different ideas.
          The first four of them can be found under this link: https://proofwiki.org/wiki/Harmonic_Series_is_Divergent



          The other four are of the following form:
          "If a subseries diverges, then the whole series diverges too. Now let's prove that the subseries of harmonic series, that formed only by reciprocals of primes diverges..."



          And there are at least four different ways to prove it: https://en.wikipedia.org/wiki/Divergence_of_the_sum_of_the_reciprocals_of_the_primes



          2) Variance of a random variable with support in $[0; 1]$ does not exceed $frac{1}{4}$



          There are at least three proofs of this, based on different ideas: What is the largest possible variance of a random variable on $[0; 1]$?



          3) No group is both a direct product and a direct product of non-trivial groups



          There are at least three proofs of this, based on different ideas: Does there exist a group that is both a free product and a direct product of nontrivial groups?



          4) $(mathbb{Q}, +)$ is not a direct product of any two non-trivial groups



          There are at least three proofs of this, based on different ideas: Is $(mathbb{Q},+)$ the direct product of two non-trivial subgroups?






          share|cite|improve this answer











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            13 Answers
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            13 Answers
            13






            active

            oldest

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            active

            oldest

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            active

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            19












            $begingroup$

            One example of a theorem with multiple proofs is the Fundamental Theorem of Algebra (All polynomials in $mathbb{C}[x]$ have the "right number" of roots). One way to prove this is build up enough complex analysis to prove that every bounded entire function is constant. Another way is to build up algebraic topology and use facts about maps from balls and circles into the punctured plane. Both of these techniques are used specifically to show one such root exists (and once this is proved the rest of the proof is easy).



            I think there are other possible proofs of the theorem but these are two I have seen.






            share|cite|improve this answer









            $endgroup$









            • 2




              $begingroup$
              See more proofs in the book The Fundamental Theorem of Algebra by Fine and Rosenberger.
              $endgroup$
              – lhf
              Jan 26 '15 at 10:33






            • 1




              $begingroup$
              You can prove the fundamental theorem of algebra using Lie theory: if $L / Bbb C$ were a finite extension, the complex projective space $PL$ would inherit the structure of a compact commutative Lie group from the multiplication on $L$. But compact commutative Lie groups are tori, and complex projective space in any positive dimension is not a torus. (Credit goes to Mariano Suárez-Alvarez; I learned this proof from a comment he posted somewhere.) You can also prove the FTA entirely algebraically, though I don't remember the details.
              $endgroup$
              – user98602
              May 6 '15 at 3:29








            • 3




              $begingroup$
              See mathoverflow.net/questions/10535/… where 44 answers have been posted.
              $endgroup$
              – Gerry Myerson
              Jul 15 '15 at 7:20






            • 1




              $begingroup$
              There is an algebraic proof that makes use of the Sylow theorems and basic Galois theory.
              $endgroup$
              – Slade
              Jul 15 '15 at 7:52
















            19












            $begingroup$

            One example of a theorem with multiple proofs is the Fundamental Theorem of Algebra (All polynomials in $mathbb{C}[x]$ have the "right number" of roots). One way to prove this is build up enough complex analysis to prove that every bounded entire function is constant. Another way is to build up algebraic topology and use facts about maps from balls and circles into the punctured plane. Both of these techniques are used specifically to show one such root exists (and once this is proved the rest of the proof is easy).



            I think there are other possible proofs of the theorem but these are two I have seen.






            share|cite|improve this answer









            $endgroup$









            • 2




              $begingroup$
              See more proofs in the book The Fundamental Theorem of Algebra by Fine and Rosenberger.
              $endgroup$
              – lhf
              Jan 26 '15 at 10:33






            • 1




              $begingroup$
              You can prove the fundamental theorem of algebra using Lie theory: if $L / Bbb C$ were a finite extension, the complex projective space $PL$ would inherit the structure of a compact commutative Lie group from the multiplication on $L$. But compact commutative Lie groups are tori, and complex projective space in any positive dimension is not a torus. (Credit goes to Mariano Suárez-Alvarez; I learned this proof from a comment he posted somewhere.) You can also prove the FTA entirely algebraically, though I don't remember the details.
              $endgroup$
              – user98602
              May 6 '15 at 3:29








            • 3




              $begingroup$
              See mathoverflow.net/questions/10535/… where 44 answers have been posted.
              $endgroup$
              – Gerry Myerson
              Jul 15 '15 at 7:20






            • 1




              $begingroup$
              There is an algebraic proof that makes use of the Sylow theorems and basic Galois theory.
              $endgroup$
              – Slade
              Jul 15 '15 at 7:52














            19












            19








            19





            $begingroup$

            One example of a theorem with multiple proofs is the Fundamental Theorem of Algebra (All polynomials in $mathbb{C}[x]$ have the "right number" of roots). One way to prove this is build up enough complex analysis to prove that every bounded entire function is constant. Another way is to build up algebraic topology and use facts about maps from balls and circles into the punctured plane. Both of these techniques are used specifically to show one such root exists (and once this is proved the rest of the proof is easy).



            I think there are other possible proofs of the theorem but these are two I have seen.






            share|cite|improve this answer









            $endgroup$



            One example of a theorem with multiple proofs is the Fundamental Theorem of Algebra (All polynomials in $mathbb{C}[x]$ have the "right number" of roots). One way to prove this is build up enough complex analysis to prove that every bounded entire function is constant. Another way is to build up algebraic topology and use facts about maps from balls and circles into the punctured plane. Both of these techniques are used specifically to show one such root exists (and once this is proved the rest of the proof is easy).



            I think there are other possible proofs of the theorem but these are two I have seen.







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Nov 8 '14 at 0:00







            user171177















            • 2




              $begingroup$
              See more proofs in the book The Fundamental Theorem of Algebra by Fine and Rosenberger.
              $endgroup$
              – lhf
              Jan 26 '15 at 10:33






            • 1




              $begingroup$
              You can prove the fundamental theorem of algebra using Lie theory: if $L / Bbb C$ were a finite extension, the complex projective space $PL$ would inherit the structure of a compact commutative Lie group from the multiplication on $L$. But compact commutative Lie groups are tori, and complex projective space in any positive dimension is not a torus. (Credit goes to Mariano Suárez-Alvarez; I learned this proof from a comment he posted somewhere.) You can also prove the FTA entirely algebraically, though I don't remember the details.
              $endgroup$
              – user98602
              May 6 '15 at 3:29








            • 3




              $begingroup$
              See mathoverflow.net/questions/10535/… where 44 answers have been posted.
              $endgroup$
              – Gerry Myerson
              Jul 15 '15 at 7:20






            • 1




              $begingroup$
              There is an algebraic proof that makes use of the Sylow theorems and basic Galois theory.
              $endgroup$
              – Slade
              Jul 15 '15 at 7:52














            • 2




              $begingroup$
              See more proofs in the book The Fundamental Theorem of Algebra by Fine and Rosenberger.
              $endgroup$
              – lhf
              Jan 26 '15 at 10:33






            • 1




              $begingroup$
              You can prove the fundamental theorem of algebra using Lie theory: if $L / Bbb C$ were a finite extension, the complex projective space $PL$ would inherit the structure of a compact commutative Lie group from the multiplication on $L$. But compact commutative Lie groups are tori, and complex projective space in any positive dimension is not a torus. (Credit goes to Mariano Suárez-Alvarez; I learned this proof from a comment he posted somewhere.) You can also prove the FTA entirely algebraically, though I don't remember the details.
              $endgroup$
              – user98602
              May 6 '15 at 3:29








            • 3




              $begingroup$
              See mathoverflow.net/questions/10535/… where 44 answers have been posted.
              $endgroup$
              – Gerry Myerson
              Jul 15 '15 at 7:20






            • 1




              $begingroup$
              There is an algebraic proof that makes use of the Sylow theorems and basic Galois theory.
              $endgroup$
              – Slade
              Jul 15 '15 at 7:52








            2




            2




            $begingroup$
            See more proofs in the book The Fundamental Theorem of Algebra by Fine and Rosenberger.
            $endgroup$
            – lhf
            Jan 26 '15 at 10:33




            $begingroup$
            See more proofs in the book The Fundamental Theorem of Algebra by Fine and Rosenberger.
            $endgroup$
            – lhf
            Jan 26 '15 at 10:33




            1




            1




            $begingroup$
            You can prove the fundamental theorem of algebra using Lie theory: if $L / Bbb C$ were a finite extension, the complex projective space $PL$ would inherit the structure of a compact commutative Lie group from the multiplication on $L$. But compact commutative Lie groups are tori, and complex projective space in any positive dimension is not a torus. (Credit goes to Mariano Suárez-Alvarez; I learned this proof from a comment he posted somewhere.) You can also prove the FTA entirely algebraically, though I don't remember the details.
            $endgroup$
            – user98602
            May 6 '15 at 3:29






            $begingroup$
            You can prove the fundamental theorem of algebra using Lie theory: if $L / Bbb C$ were a finite extension, the complex projective space $PL$ would inherit the structure of a compact commutative Lie group from the multiplication on $L$. But compact commutative Lie groups are tori, and complex projective space in any positive dimension is not a torus. (Credit goes to Mariano Suárez-Alvarez; I learned this proof from a comment he posted somewhere.) You can also prove the FTA entirely algebraically, though I don't remember the details.
            $endgroup$
            – user98602
            May 6 '15 at 3:29






            3




            3




            $begingroup$
            See mathoverflow.net/questions/10535/… where 44 answers have been posted.
            $endgroup$
            – Gerry Myerson
            Jul 15 '15 at 7:20




            $begingroup$
            See mathoverflow.net/questions/10535/… where 44 answers have been posted.
            $endgroup$
            – Gerry Myerson
            Jul 15 '15 at 7:20




            1




            1




            $begingroup$
            There is an algebraic proof that makes use of the Sylow theorems and basic Galois theory.
            $endgroup$
            – Slade
            Jul 15 '15 at 7:52




            $begingroup$
            There is an algebraic proof that makes use of the Sylow theorems and basic Galois theory.
            $endgroup$
            – Slade
            Jul 15 '15 at 7:52











            15












            $begingroup$

            The Brouwer fixed-point theorem. The Wikipedia lists the following methods:




            • Homology

            • Stokes's theorem

            • Combinatorial (Sperner's lemma)

            • Reducing to the smooth case and using Sard's theorem

            • Reducing to the smooth case and using the COV theorem

            • Lefschetz fixed-point theorem

            • Using Hex






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              What is COV theorem?
              $endgroup$
              – Mizar
              Jan 28 '15 at 9:10










            • $begingroup$
              @Mizar, change of variables: en.wikipedia.org/wiki/….
              $endgroup$
              – Martín-Blas Pérez Pinilla
              Jan 28 '15 at 9:14












            • $begingroup$
              While we're at it, there are two proofs using the Brouwer degree for continuous maps of open sets in $mathbb{R}^n$. One is essentially the same as Hirsch's theorem based on the impossibility of a retraction onto $S^{n-1}$, and the other constructs a homotopy between the identity and identity minus the continuous map.
              $endgroup$
              – Gyu Eun Lee
              Mar 16 '15 at 7:14
















            15












            $begingroup$

            The Brouwer fixed-point theorem. The Wikipedia lists the following methods:




            • Homology

            • Stokes's theorem

            • Combinatorial (Sperner's lemma)

            • Reducing to the smooth case and using Sard's theorem

            • Reducing to the smooth case and using the COV theorem

            • Lefschetz fixed-point theorem

            • Using Hex






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              What is COV theorem?
              $endgroup$
              – Mizar
              Jan 28 '15 at 9:10










            • $begingroup$
              @Mizar, change of variables: en.wikipedia.org/wiki/….
              $endgroup$
              – Martín-Blas Pérez Pinilla
              Jan 28 '15 at 9:14












            • $begingroup$
              While we're at it, there are two proofs using the Brouwer degree for continuous maps of open sets in $mathbb{R}^n$. One is essentially the same as Hirsch's theorem based on the impossibility of a retraction onto $S^{n-1}$, and the other constructs a homotopy between the identity and identity minus the continuous map.
              $endgroup$
              – Gyu Eun Lee
              Mar 16 '15 at 7:14














            15












            15








            15





            $begingroup$

            The Brouwer fixed-point theorem. The Wikipedia lists the following methods:




            • Homology

            • Stokes's theorem

            • Combinatorial (Sperner's lemma)

            • Reducing to the smooth case and using Sard's theorem

            • Reducing to the smooth case and using the COV theorem

            • Lefschetz fixed-point theorem

            • Using Hex






            share|cite|improve this answer









            $endgroup$



            The Brouwer fixed-point theorem. The Wikipedia lists the following methods:




            • Homology

            • Stokes's theorem

            • Combinatorial (Sperner's lemma)

            • Reducing to the smooth case and using Sard's theorem

            • Reducing to the smooth case and using the COV theorem

            • Lefschetz fixed-point theorem

            • Using Hex







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Jan 26 '15 at 10:15









            Martín-Blas Pérez PinillaMartín-Blas Pérez Pinilla

            35.5k42972




            35.5k42972












            • $begingroup$
              What is COV theorem?
              $endgroup$
              – Mizar
              Jan 28 '15 at 9:10










            • $begingroup$
              @Mizar, change of variables: en.wikipedia.org/wiki/….
              $endgroup$
              – Martín-Blas Pérez Pinilla
              Jan 28 '15 at 9:14












            • $begingroup$
              While we're at it, there are two proofs using the Brouwer degree for continuous maps of open sets in $mathbb{R}^n$. One is essentially the same as Hirsch's theorem based on the impossibility of a retraction onto $S^{n-1}$, and the other constructs a homotopy between the identity and identity minus the continuous map.
              $endgroup$
              – Gyu Eun Lee
              Mar 16 '15 at 7:14


















            • $begingroup$
              What is COV theorem?
              $endgroup$
              – Mizar
              Jan 28 '15 at 9:10










            • $begingroup$
              @Mizar, change of variables: en.wikipedia.org/wiki/….
              $endgroup$
              – Martín-Blas Pérez Pinilla
              Jan 28 '15 at 9:14












            • $begingroup$
              While we're at it, there are two proofs using the Brouwer degree for continuous maps of open sets in $mathbb{R}^n$. One is essentially the same as Hirsch's theorem based on the impossibility of a retraction onto $S^{n-1}$, and the other constructs a homotopy between the identity and identity minus the continuous map.
              $endgroup$
              – Gyu Eun Lee
              Mar 16 '15 at 7:14
















            $begingroup$
            What is COV theorem?
            $endgroup$
            – Mizar
            Jan 28 '15 at 9:10




            $begingroup$
            What is COV theorem?
            $endgroup$
            – Mizar
            Jan 28 '15 at 9:10












            $begingroup$
            @Mizar, change of variables: en.wikipedia.org/wiki/….
            $endgroup$
            – Martín-Blas Pérez Pinilla
            Jan 28 '15 at 9:14






            $begingroup$
            @Mizar, change of variables: en.wikipedia.org/wiki/….
            $endgroup$
            – Martín-Blas Pérez Pinilla
            Jan 28 '15 at 9:14














            $begingroup$
            While we're at it, there are two proofs using the Brouwer degree for continuous maps of open sets in $mathbb{R}^n$. One is essentially the same as Hirsch's theorem based on the impossibility of a retraction onto $S^{n-1}$, and the other constructs a homotopy between the identity and identity minus the continuous map.
            $endgroup$
            – Gyu Eun Lee
            Mar 16 '15 at 7:14




            $begingroup$
            While we're at it, there are two proofs using the Brouwer degree for continuous maps of open sets in $mathbb{R}^n$. One is essentially the same as Hirsch's theorem based on the impossibility of a retraction onto $S^{n-1}$, and the other constructs a homotopy between the identity and identity minus the continuous map.
            $endgroup$
            – Gyu Eun Lee
            Mar 16 '15 at 7:14











            14












            $begingroup$

            A large rectangle is tiled with smaller rectangles. Each of the smaller rectangles has at least one integer side. Must the large rectangle have at least one integer side?



            What if you replace 'integer' with 'rational' or 'algebraic'?



            Here are fourteen proofs.






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              Thanks, this is very nice!
              $endgroup$
              – joriki
              Jul 27 '15 at 17:29
















            14












            $begingroup$

            A large rectangle is tiled with smaller rectangles. Each of the smaller rectangles has at least one integer side. Must the large rectangle have at least one integer side?



            What if you replace 'integer' with 'rational' or 'algebraic'?



            Here are fourteen proofs.






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              Thanks, this is very nice!
              $endgroup$
              – joriki
              Jul 27 '15 at 17:29














            14












            14








            14





            $begingroup$

            A large rectangle is tiled with smaller rectangles. Each of the smaller rectangles has at least one integer side. Must the large rectangle have at least one integer side?



            What if you replace 'integer' with 'rational' or 'algebraic'?



            Here are fourteen proofs.






            share|cite|improve this answer









            $endgroup$



            A large rectangle is tiled with smaller rectangles. Each of the smaller rectangles has at least one integer side. Must the large rectangle have at least one integer side?



            What if you replace 'integer' with 'rational' or 'algebraic'?



            Here are fourteen proofs.







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Jan 18 '15 at 13:10









            LopsyLopsy

            3,7731423




            3,7731423












            • $begingroup$
              Thanks, this is very nice!
              $endgroup$
              – joriki
              Jul 27 '15 at 17:29


















            • $begingroup$
              Thanks, this is very nice!
              $endgroup$
              – joriki
              Jul 27 '15 at 17:29
















            $begingroup$
            Thanks, this is very nice!
            $endgroup$
            – joriki
            Jul 27 '15 at 17:29




            $begingroup$
            Thanks, this is very nice!
            $endgroup$
            – joriki
            Jul 27 '15 at 17:29











            14












            $begingroup$

            As a kind of extreme example, I've heard of this book, reputed to have given over 360 proofs of the Pythagorean theorem. I'm not sure how pairwise different they are, but there's bound to be a lot of variety.



            Another one that sticks in my mind is the proof of the Cayley-Hamilton theorem
            for real (or complex) matrices. There is an algebraic proof that works for any field at all, but for the reals and complexes, you can argue topologically that the matrix you are working with is the limit of a sequence of diagonalizable matrices, and since the approximations satisfy their characteristic polynomial, so does the limit.






            share|cite|improve this answer











            $endgroup$









            • 3




              $begingroup$
              +1, and interestingly, the geometric proof of Cayley-Hamilton even carries over to arbitrary fields when one uses the Zariski topology.
              $endgroup$
              – Hanno
              Jan 18 '15 at 13:15






            • 1




              $begingroup$
              I'm currently this this, which gives a number of different proofs of the Pythagorean Theorem. Some proofs are very similar to each other, but others are quite different.
              $endgroup$
              – Randy E
              Jan 18 '15 at 13:16


















            14












            $begingroup$

            As a kind of extreme example, I've heard of this book, reputed to have given over 360 proofs of the Pythagorean theorem. I'm not sure how pairwise different they are, but there's bound to be a lot of variety.



            Another one that sticks in my mind is the proof of the Cayley-Hamilton theorem
            for real (or complex) matrices. There is an algebraic proof that works for any field at all, but for the reals and complexes, you can argue topologically that the matrix you are working with is the limit of a sequence of diagonalizable matrices, and since the approximations satisfy their characteristic polynomial, so does the limit.






            share|cite|improve this answer











            $endgroup$









            • 3




              $begingroup$
              +1, and interestingly, the geometric proof of Cayley-Hamilton even carries over to arbitrary fields when one uses the Zariski topology.
              $endgroup$
              – Hanno
              Jan 18 '15 at 13:15






            • 1




              $begingroup$
              I'm currently this this, which gives a number of different proofs of the Pythagorean Theorem. Some proofs are very similar to each other, but others are quite different.
              $endgroup$
              – Randy E
              Jan 18 '15 at 13:16
















            14












            14








            14





            $begingroup$

            As a kind of extreme example, I've heard of this book, reputed to have given over 360 proofs of the Pythagorean theorem. I'm not sure how pairwise different they are, but there's bound to be a lot of variety.



            Another one that sticks in my mind is the proof of the Cayley-Hamilton theorem
            for real (or complex) matrices. There is an algebraic proof that works for any field at all, but for the reals and complexes, you can argue topologically that the matrix you are working with is the limit of a sequence of diagonalizable matrices, and since the approximations satisfy their characteristic polynomial, so does the limit.






            share|cite|improve this answer











            $endgroup$



            As a kind of extreme example, I've heard of this book, reputed to have given over 360 proofs of the Pythagorean theorem. I'm not sure how pairwise different they are, but there's bound to be a lot of variety.



            Another one that sticks in my mind is the proof of the Cayley-Hamilton theorem
            for real (or complex) matrices. There is an algebraic proof that works for any field at all, but for the reals and complexes, you can argue topologically that the matrix you are working with is the limit of a sequence of diagonalizable matrices, and since the approximations satisfy their characteristic polynomial, so does the limit.







            share|cite|improve this answer














            share|cite|improve this answer



            share|cite|improve this answer








            edited Jul 15 '15 at 6:58









            Bumblebee

            9,74912551




            9,74912551










            answered Jan 18 '15 at 12:59









            rschwiebrschwieb

            108k12104253




            108k12104253








            • 3




              $begingroup$
              +1, and interestingly, the geometric proof of Cayley-Hamilton even carries over to arbitrary fields when one uses the Zariski topology.
              $endgroup$
              – Hanno
              Jan 18 '15 at 13:15






            • 1




              $begingroup$
              I'm currently this this, which gives a number of different proofs of the Pythagorean Theorem. Some proofs are very similar to each other, but others are quite different.
              $endgroup$
              – Randy E
              Jan 18 '15 at 13:16
















            • 3




              $begingroup$
              +1, and interestingly, the geometric proof of Cayley-Hamilton even carries over to arbitrary fields when one uses the Zariski topology.
              $endgroup$
              – Hanno
              Jan 18 '15 at 13:15






            • 1




              $begingroup$
              I'm currently this this, which gives a number of different proofs of the Pythagorean Theorem. Some proofs are very similar to each other, but others are quite different.
              $endgroup$
              – Randy E
              Jan 18 '15 at 13:16










            3




            3




            $begingroup$
            +1, and interestingly, the geometric proof of Cayley-Hamilton even carries over to arbitrary fields when one uses the Zariski topology.
            $endgroup$
            – Hanno
            Jan 18 '15 at 13:15




            $begingroup$
            +1, and interestingly, the geometric proof of Cayley-Hamilton even carries over to arbitrary fields when one uses the Zariski topology.
            $endgroup$
            – Hanno
            Jan 18 '15 at 13:15




            1




            1




            $begingroup$
            I'm currently this this, which gives a number of different proofs of the Pythagorean Theorem. Some proofs are very similar to each other, but others are quite different.
            $endgroup$
            – Randy E
            Jan 18 '15 at 13:16






            $begingroup$
            I'm currently this this, which gives a number of different proofs of the Pythagorean Theorem. Some proofs are very similar to each other, but others are quite different.
            $endgroup$
            – Randy E
            Jan 18 '15 at 13:16













            11












            $begingroup$


            1. $20$ different proofs 0f the Euler Formula
              $$color{Red}{V-E+F=2.}$$


            2. $10$ different proofs 0f the Gaussian Integral
              $$color{Green}{int_{mathbb{R}}e^{-x^2},mathrm{d}x=sqrtpi.}$$







            share|cite|improve this answer











            $endgroup$













            • $begingroup$
              This and this also provides some methods of evaluating Gaussian integral.
              $endgroup$
              – Bumblebee
              Jul 15 '15 at 8:17
















            11












            $begingroup$


            1. $20$ different proofs 0f the Euler Formula
              $$color{Red}{V-E+F=2.}$$


            2. $10$ different proofs 0f the Gaussian Integral
              $$color{Green}{int_{mathbb{R}}e^{-x^2},mathrm{d}x=sqrtpi.}$$







            share|cite|improve this answer











            $endgroup$













            • $begingroup$
              This and this also provides some methods of evaluating Gaussian integral.
              $endgroup$
              – Bumblebee
              Jul 15 '15 at 8:17














            11












            11








            11





            $begingroup$


            1. $20$ different proofs 0f the Euler Formula
              $$color{Red}{V-E+F=2.}$$


            2. $10$ different proofs 0f the Gaussian Integral
              $$color{Green}{int_{mathbb{R}}e^{-x^2},mathrm{d}x=sqrtpi.}$$







            share|cite|improve this answer











            $endgroup$




            1. $20$ different proofs 0f the Euler Formula
              $$color{Red}{V-E+F=2.}$$


            2. $10$ different proofs 0f the Gaussian Integral
              $$color{Green}{int_{mathbb{R}}e^{-x^2},mathrm{d}x=sqrtpi.}$$








            share|cite|improve this answer














            share|cite|improve this answer



            share|cite|improve this answer








            edited Jan 30 '17 at 16:02

























            answered Jan 28 '15 at 8:56









            BumblebeeBumblebee

            9,74912551




            9,74912551












            • $begingroup$
              This and this also provides some methods of evaluating Gaussian integral.
              $endgroup$
              – Bumblebee
              Jul 15 '15 at 8:17


















            • $begingroup$
              This and this also provides some methods of evaluating Gaussian integral.
              $endgroup$
              – Bumblebee
              Jul 15 '15 at 8:17
















            $begingroup$
            This and this also provides some methods of evaluating Gaussian integral.
            $endgroup$
            – Bumblebee
            Jul 15 '15 at 8:17




            $begingroup$
            This and this also provides some methods of evaluating Gaussian integral.
            $endgroup$
            – Bumblebee
            Jul 15 '15 at 8:17











            10












            $begingroup$

            A famous example is the law of quadratic reciprocity. Wikipedia says that
            several hundred proofs of the law of quadratic reciprocity have been found.
            Many details concerning proofs by different methods can be found at the question at MO.






            share|cite|improve this answer











            $endgroup$


















              10












              $begingroup$

              A famous example is the law of quadratic reciprocity. Wikipedia says that
              several hundred proofs of the law of quadratic reciprocity have been found.
              Many details concerning proofs by different methods can be found at the question at MO.






              share|cite|improve this answer











              $endgroup$
















                10












                10








                10





                $begingroup$

                A famous example is the law of quadratic reciprocity. Wikipedia says that
                several hundred proofs of the law of quadratic reciprocity have been found.
                Many details concerning proofs by different methods can be found at the question at MO.






                share|cite|improve this answer











                $endgroup$



                A famous example is the law of quadratic reciprocity. Wikipedia says that
                several hundred proofs of the law of quadratic reciprocity have been found.
                Many details concerning proofs by different methods can be found at the question at MO.







                share|cite|improve this answer














                share|cite|improve this answer



                share|cite|improve this answer








                edited Apr 13 '17 at 12:58









                Community

                1




                1










                answered Apr 29 '15 at 8:06









                Dietrich BurdeDietrich Burde

                82.1k649107




                82.1k649107























                    6












                    $begingroup$

                    The first example that comes to my mind is the Cauchy-Schwarz inequality.



                    Here there is a paper I found some time ago in which the authors claim (I write claim, because I did not check each proof) to be able to show twelve different proofs of the result.



                    Actually, there is an entire book on inequalities that starts from this very basic one, with various proofs of it.






                    share|cite|improve this answer









                    $endgroup$


















                      6












                      $begingroup$

                      The first example that comes to my mind is the Cauchy-Schwarz inequality.



                      Here there is a paper I found some time ago in which the authors claim (I write claim, because I did not check each proof) to be able to show twelve different proofs of the result.



                      Actually, there is an entire book on inequalities that starts from this very basic one, with various proofs of it.






                      share|cite|improve this answer









                      $endgroup$
















                        6












                        6








                        6





                        $begingroup$

                        The first example that comes to my mind is the Cauchy-Schwarz inequality.



                        Here there is a paper I found some time ago in which the authors claim (I write claim, because I did not check each proof) to be able to show twelve different proofs of the result.



                        Actually, there is an entire book on inequalities that starts from this very basic one, with various proofs of it.






                        share|cite|improve this answer









                        $endgroup$



                        The first example that comes to my mind is the Cauchy-Schwarz inequality.



                        Here there is a paper I found some time ago in which the authors claim (I write claim, because I did not check each proof) to be able to show twelve different proofs of the result.



                        Actually, there is an entire book on inequalities that starts from this very basic one, with various proofs of it.







                        share|cite|improve this answer












                        share|cite|improve this answer



                        share|cite|improve this answer










                        answered Jan 28 '15 at 9:11









                        KolminKolmin

                        1,89411435




                        1,89411435























                            5












                            $begingroup$

                            There are several results of this kind: For example the period three implies chaos theorem (http://mathworld.wolfram.com/PeriodThreeTheorem.html) was proved by Li and York (in 1975) by using the intermediate values theorem for continuous functions in despite the result belongs to discrete dynamics. A generalization of this result (http://mathworld.wolfram.com/SharkovskysTheorem.html) was made before (in 1965) by A.N. Sharkovsky by using a very complicated and different method.



                            Also, I note that the most know theorems of L. Riemann were proved by introducing new ideas based on complex analysis. Those of Euler are based on the series...etc.






                            share|cite|improve this answer











                            $endgroup$


















                              5












                              $begingroup$

                              There are several results of this kind: For example the period three implies chaos theorem (http://mathworld.wolfram.com/PeriodThreeTheorem.html) was proved by Li and York (in 1975) by using the intermediate values theorem for continuous functions in despite the result belongs to discrete dynamics. A generalization of this result (http://mathworld.wolfram.com/SharkovskysTheorem.html) was made before (in 1965) by A.N. Sharkovsky by using a very complicated and different method.



                              Also, I note that the most know theorems of L. Riemann were proved by introducing new ideas based on complex analysis. Those of Euler are based on the series...etc.






                              share|cite|improve this answer











                              $endgroup$
















                                5












                                5








                                5





                                $begingroup$

                                There are several results of this kind: For example the period three implies chaos theorem (http://mathworld.wolfram.com/PeriodThreeTheorem.html) was proved by Li and York (in 1975) by using the intermediate values theorem for continuous functions in despite the result belongs to discrete dynamics. A generalization of this result (http://mathworld.wolfram.com/SharkovskysTheorem.html) was made before (in 1965) by A.N. Sharkovsky by using a very complicated and different method.



                                Also, I note that the most know theorems of L. Riemann were proved by introducing new ideas based on complex analysis. Those of Euler are based on the series...etc.






                                share|cite|improve this answer











                                $endgroup$



                                There are several results of this kind: For example the period three implies chaos theorem (http://mathworld.wolfram.com/PeriodThreeTheorem.html) was proved by Li and York (in 1975) by using the intermediate values theorem for continuous functions in despite the result belongs to discrete dynamics. A generalization of this result (http://mathworld.wolfram.com/SharkovskysTheorem.html) was made before (in 1965) by A.N. Sharkovsky by using a very complicated and different method.



                                Also, I note that the most know theorems of L. Riemann were proved by introducing new ideas based on complex analysis. Those of Euler are based on the series...etc.







                                share|cite|improve this answer














                                share|cite|improve this answer



                                share|cite|improve this answer








                                edited Jan 18 '15 at 13:04

























                                answered Jan 18 '15 at 12:48









                                DERDER

                                1,6631018




                                1,6631018























                                    4












                                    $begingroup$

                                    Irving Adler in 'a new look at geometry' gives a circular chain of proofs of alternate versions of the fifth postulate (that if a line striking each of two lines at the same angle, then the two lines do not cross at any distance).



                                    The idea of such a proof shows that all of these propositions are identical in function.






                                    share|cite|improve this answer









                                    $endgroup$


















                                      4












                                      $begingroup$

                                      Irving Adler in 'a new look at geometry' gives a circular chain of proofs of alternate versions of the fifth postulate (that if a line striking each of two lines at the same angle, then the two lines do not cross at any distance).



                                      The idea of such a proof shows that all of these propositions are identical in function.






                                      share|cite|improve this answer









                                      $endgroup$
















                                        4












                                        4








                                        4





                                        $begingroup$

                                        Irving Adler in 'a new look at geometry' gives a circular chain of proofs of alternate versions of the fifth postulate (that if a line striking each of two lines at the same angle, then the two lines do not cross at any distance).



                                        The idea of such a proof shows that all of these propositions are identical in function.






                                        share|cite|improve this answer









                                        $endgroup$



                                        Irving Adler in 'a new look at geometry' gives a circular chain of proofs of alternate versions of the fifth postulate (that if a line striking each of two lines at the same angle, then the two lines do not cross at any distance).



                                        The idea of such a proof shows that all of these propositions are identical in function.







                                        share|cite|improve this answer












                                        share|cite|improve this answer



                                        share|cite|improve this answer










                                        answered Jan 28 '15 at 9:33









                                        wendy.kriegerwendy.krieger

                                        5,86311427




                                        5,86311427























                                            4












                                            $begingroup$

                                            Proofs that $sqrt{2}$
                                            is irrational.



                                            I know (at least) four,
                                            and there are lists of many more.
                                            I could go on,
                                            but would be a bore,
                                            so I'll let someone else
                                            have the floor.






                                            share|cite|improve this answer









                                            $endgroup$









                                            • 1




                                              $begingroup$
                                              Do a Google search for "proof square root of 2 is irrational". One link, which has a number of proofs, is here: en.wikipedia.org/wiki/Square_root_of_2
                                              $endgroup$
                                              – marty cohen
                                              May 22 '15 at 14:56










                                            • $begingroup$
                                              I found your this post. Thank you.
                                              $endgroup$
                                              – Bumblebee
                                              Jun 22 '15 at 7:43






                                            • 1




                                              $begingroup$
                                              Another link with may proofs is cut-the-knot.org/proofs/sq_root.shtml
                                              $endgroup$
                                              – Gerry Myerson
                                              Jul 21 '15 at 6:56






                                            • 1




                                              $begingroup$
                                              When writing your answer, did you mean to rhyme each time [: )]?
                                              $endgroup$
                                              – Antonio DJC
                                              Jun 11 '17 at 3:33










                                            • $begingroup$
                                              One thing is certain: my rhyming is rarely inadverten t.
                                              $endgroup$
                                              – marty cohen
                                              Jun 11 '17 at 4:49
















                                            4












                                            $begingroup$

                                            Proofs that $sqrt{2}$
                                            is irrational.



                                            I know (at least) four,
                                            and there are lists of many more.
                                            I could go on,
                                            but would be a bore,
                                            so I'll let someone else
                                            have the floor.






                                            share|cite|improve this answer









                                            $endgroup$









                                            • 1




                                              $begingroup$
                                              Do a Google search for "proof square root of 2 is irrational". One link, which has a number of proofs, is here: en.wikipedia.org/wiki/Square_root_of_2
                                              $endgroup$
                                              – marty cohen
                                              May 22 '15 at 14:56










                                            • $begingroup$
                                              I found your this post. Thank you.
                                              $endgroup$
                                              – Bumblebee
                                              Jun 22 '15 at 7:43






                                            • 1




                                              $begingroup$
                                              Another link with may proofs is cut-the-knot.org/proofs/sq_root.shtml
                                              $endgroup$
                                              – Gerry Myerson
                                              Jul 21 '15 at 6:56






                                            • 1




                                              $begingroup$
                                              When writing your answer, did you mean to rhyme each time [: )]?
                                              $endgroup$
                                              – Antonio DJC
                                              Jun 11 '17 at 3:33










                                            • $begingroup$
                                              One thing is certain: my rhyming is rarely inadverten t.
                                              $endgroup$
                                              – marty cohen
                                              Jun 11 '17 at 4:49














                                            4












                                            4








                                            4





                                            $begingroup$

                                            Proofs that $sqrt{2}$
                                            is irrational.



                                            I know (at least) four,
                                            and there are lists of many more.
                                            I could go on,
                                            but would be a bore,
                                            so I'll let someone else
                                            have the floor.






                                            share|cite|improve this answer









                                            $endgroup$



                                            Proofs that $sqrt{2}$
                                            is irrational.



                                            I know (at least) four,
                                            and there are lists of many more.
                                            I could go on,
                                            but would be a bore,
                                            so I'll let someone else
                                            have the floor.







                                            share|cite|improve this answer












                                            share|cite|improve this answer



                                            share|cite|improve this answer










                                            answered May 6 '15 at 3:50









                                            marty cohenmarty cohen

                                            75.7k549130




                                            75.7k549130








                                            • 1




                                              $begingroup$
                                              Do a Google search for "proof square root of 2 is irrational". One link, which has a number of proofs, is here: en.wikipedia.org/wiki/Square_root_of_2
                                              $endgroup$
                                              – marty cohen
                                              May 22 '15 at 14:56










                                            • $begingroup$
                                              I found your this post. Thank you.
                                              $endgroup$
                                              – Bumblebee
                                              Jun 22 '15 at 7:43






                                            • 1




                                              $begingroup$
                                              Another link with may proofs is cut-the-knot.org/proofs/sq_root.shtml
                                              $endgroup$
                                              – Gerry Myerson
                                              Jul 21 '15 at 6:56






                                            • 1




                                              $begingroup$
                                              When writing your answer, did you mean to rhyme each time [: )]?
                                              $endgroup$
                                              – Antonio DJC
                                              Jun 11 '17 at 3:33










                                            • $begingroup$
                                              One thing is certain: my rhyming is rarely inadverten t.
                                              $endgroup$
                                              – marty cohen
                                              Jun 11 '17 at 4:49














                                            • 1




                                              $begingroup$
                                              Do a Google search for "proof square root of 2 is irrational". One link, which has a number of proofs, is here: en.wikipedia.org/wiki/Square_root_of_2
                                              $endgroup$
                                              – marty cohen
                                              May 22 '15 at 14:56










                                            • $begingroup$
                                              I found your this post. Thank you.
                                              $endgroup$
                                              – Bumblebee
                                              Jun 22 '15 at 7:43






                                            • 1




                                              $begingroup$
                                              Another link with may proofs is cut-the-knot.org/proofs/sq_root.shtml
                                              $endgroup$
                                              – Gerry Myerson
                                              Jul 21 '15 at 6:56






                                            • 1




                                              $begingroup$
                                              When writing your answer, did you mean to rhyme each time [: )]?
                                              $endgroup$
                                              – Antonio DJC
                                              Jun 11 '17 at 3:33










                                            • $begingroup$
                                              One thing is certain: my rhyming is rarely inadverten t.
                                              $endgroup$
                                              – marty cohen
                                              Jun 11 '17 at 4:49








                                            1




                                            1




                                            $begingroup$
                                            Do a Google search for "proof square root of 2 is irrational". One link, which has a number of proofs, is here: en.wikipedia.org/wiki/Square_root_of_2
                                            $endgroup$
                                            – marty cohen
                                            May 22 '15 at 14:56




                                            $begingroup$
                                            Do a Google search for "proof square root of 2 is irrational". One link, which has a number of proofs, is here: en.wikipedia.org/wiki/Square_root_of_2
                                            $endgroup$
                                            – marty cohen
                                            May 22 '15 at 14:56












                                            $begingroup$
                                            I found your this post. Thank you.
                                            $endgroup$
                                            – Bumblebee
                                            Jun 22 '15 at 7:43




                                            $begingroup$
                                            I found your this post. Thank you.
                                            $endgroup$
                                            – Bumblebee
                                            Jun 22 '15 at 7:43




                                            1




                                            1




                                            $begingroup$
                                            Another link with may proofs is cut-the-knot.org/proofs/sq_root.shtml
                                            $endgroup$
                                            – Gerry Myerson
                                            Jul 21 '15 at 6:56




                                            $begingroup$
                                            Another link with may proofs is cut-the-knot.org/proofs/sq_root.shtml
                                            $endgroup$
                                            – Gerry Myerson
                                            Jul 21 '15 at 6:56




                                            1




                                            1




                                            $begingroup$
                                            When writing your answer, did you mean to rhyme each time [: )]?
                                            $endgroup$
                                            – Antonio DJC
                                            Jun 11 '17 at 3:33




                                            $begingroup$
                                            When writing your answer, did you mean to rhyme each time [: )]?
                                            $endgroup$
                                            – Antonio DJC
                                            Jun 11 '17 at 3:33












                                            $begingroup$
                                            One thing is certain: my rhyming is rarely inadverten t.
                                            $endgroup$
                                            – marty cohen
                                            Jun 11 '17 at 4:49




                                            $begingroup$
                                            One thing is certain: my rhyming is rarely inadverten t.
                                            $endgroup$
                                            – marty cohen
                                            Jun 11 '17 at 4:49











                                            4












                                            $begingroup$

                                            Robin Chapman gives 14 proofs of $$sum_{n=1}^{infty}{1over n^2}={pi^2over6}$$ here.






                                            share|cite|improve this answer









                                            $endgroup$













                                            • $begingroup$
                                              This article provides a beautiful non-proof (Euler's proof) of this identity. Thank you.
                                              $endgroup$
                                              – Bumblebee
                                              Jul 17 '15 at 8:50


















                                            4












                                            $begingroup$

                                            Robin Chapman gives 14 proofs of $$sum_{n=1}^{infty}{1over n^2}={pi^2over6}$$ here.






                                            share|cite|improve this answer









                                            $endgroup$













                                            • $begingroup$
                                              This article provides a beautiful non-proof (Euler's proof) of this identity. Thank you.
                                              $endgroup$
                                              – Bumblebee
                                              Jul 17 '15 at 8:50
















                                            4












                                            4








                                            4





                                            $begingroup$

                                            Robin Chapman gives 14 proofs of $$sum_{n=1}^{infty}{1over n^2}={pi^2over6}$$ here.






                                            share|cite|improve this answer









                                            $endgroup$



                                            Robin Chapman gives 14 proofs of $$sum_{n=1}^{infty}{1over n^2}={pi^2over6}$$ here.







                                            share|cite|improve this answer












                                            share|cite|improve this answer



                                            share|cite|improve this answer










                                            answered Jul 15 '15 at 7:22









                                            Gerry MyersonGerry Myerson

                                            148k8152306




                                            148k8152306












                                            • $begingroup$
                                              This article provides a beautiful non-proof (Euler's proof) of this identity. Thank you.
                                              $endgroup$
                                              – Bumblebee
                                              Jul 17 '15 at 8:50




















                                            • $begingroup$
                                              This article provides a beautiful non-proof (Euler's proof) of this identity. Thank you.
                                              $endgroup$
                                              – Bumblebee
                                              Jul 17 '15 at 8:50


















                                            $begingroup$
                                            This article provides a beautiful non-proof (Euler's proof) of this identity. Thank you.
                                            $endgroup$
                                            – Bumblebee
                                            Jul 17 '15 at 8:50






                                            $begingroup$
                                            This article provides a beautiful non-proof (Euler's proof) of this identity. Thank you.
                                            $endgroup$
                                            – Bumblebee
                                            Jul 17 '15 at 8:50













                                            3












                                            $begingroup$

                                            For $ngeq kinmathbb{N}$, you can prove that $dfrac{n!}{k!cdot(n-k)!}$ is an integer from a combinatorial/counting argument. Establish that the formula gives the number of ways to choose a subset of $k$ from a set of $n$, and that automatically makes it an integer.



                                            Or you can prove that $dfrac{n!}{k!cdot(n-k)!}$ is an integer by showing that the highest power of any prime $p$ dividing the denominator is less than the highest power of $p$ dividing the numerator.



                                            Several proofs here.






                                            share|cite|improve this answer











                                            $endgroup$









                                            • 3




                                              $begingroup$
                                              You can also prove this using group actions: $S_n$ acts on the set ${1, 2, dotsc, n}$, and the stabilizer of any $k$-set from that set is $S_ktimes S_{n-k}$. So by the orbit-stabilizer theorem, the number of such $k$-sets is the number of orbits, which is $frac{|S_n|}{|S_k||S_{n-k}|} = frac{n!}{k!(n-k)!}$.
                                              $endgroup$
                                              – rogerl
                                              Jul 27 '15 at 17:27
















                                            3












                                            $begingroup$

                                            For $ngeq kinmathbb{N}$, you can prove that $dfrac{n!}{k!cdot(n-k)!}$ is an integer from a combinatorial/counting argument. Establish that the formula gives the number of ways to choose a subset of $k$ from a set of $n$, and that automatically makes it an integer.



                                            Or you can prove that $dfrac{n!}{k!cdot(n-k)!}$ is an integer by showing that the highest power of any prime $p$ dividing the denominator is less than the highest power of $p$ dividing the numerator.



                                            Several proofs here.






                                            share|cite|improve this answer











                                            $endgroup$









                                            • 3




                                              $begingroup$
                                              You can also prove this using group actions: $S_n$ acts on the set ${1, 2, dotsc, n}$, and the stabilizer of any $k$-set from that set is $S_ktimes S_{n-k}$. So by the orbit-stabilizer theorem, the number of such $k$-sets is the number of orbits, which is $frac{|S_n|}{|S_k||S_{n-k}|} = frac{n!}{k!(n-k)!}$.
                                              $endgroup$
                                              – rogerl
                                              Jul 27 '15 at 17:27














                                            3












                                            3








                                            3





                                            $begingroup$

                                            For $ngeq kinmathbb{N}$, you can prove that $dfrac{n!}{k!cdot(n-k)!}$ is an integer from a combinatorial/counting argument. Establish that the formula gives the number of ways to choose a subset of $k$ from a set of $n$, and that automatically makes it an integer.



                                            Or you can prove that $dfrac{n!}{k!cdot(n-k)!}$ is an integer by showing that the highest power of any prime $p$ dividing the denominator is less than the highest power of $p$ dividing the numerator.



                                            Several proofs here.






                                            share|cite|improve this answer











                                            $endgroup$



                                            For $ngeq kinmathbb{N}$, you can prove that $dfrac{n!}{k!cdot(n-k)!}$ is an integer from a combinatorial/counting argument. Establish that the formula gives the number of ways to choose a subset of $k$ from a set of $n$, and that automatically makes it an integer.



                                            Or you can prove that $dfrac{n!}{k!cdot(n-k)!}$ is an integer by showing that the highest power of any prime $p$ dividing the denominator is less than the highest power of $p$ dividing the numerator.



                                            Several proofs here.







                                            share|cite|improve this answer














                                            share|cite|improve this answer



                                            share|cite|improve this answer








                                            edited Apr 13 '17 at 12:20









                                            Community

                                            1




                                            1










                                            answered Jul 27 '15 at 16:40









                                            alex.jordanalex.jordan

                                            39.7k660122




                                            39.7k660122








                                            • 3




                                              $begingroup$
                                              You can also prove this using group actions: $S_n$ acts on the set ${1, 2, dotsc, n}$, and the stabilizer of any $k$-set from that set is $S_ktimes S_{n-k}$. So by the orbit-stabilizer theorem, the number of such $k$-sets is the number of orbits, which is $frac{|S_n|}{|S_k||S_{n-k}|} = frac{n!}{k!(n-k)!}$.
                                              $endgroup$
                                              – rogerl
                                              Jul 27 '15 at 17:27














                                            • 3




                                              $begingroup$
                                              You can also prove this using group actions: $S_n$ acts on the set ${1, 2, dotsc, n}$, and the stabilizer of any $k$-set from that set is $S_ktimes S_{n-k}$. So by the orbit-stabilizer theorem, the number of such $k$-sets is the number of orbits, which is $frac{|S_n|}{|S_k||S_{n-k}|} = frac{n!}{k!(n-k)!}$.
                                              $endgroup$
                                              – rogerl
                                              Jul 27 '15 at 17:27








                                            3




                                            3




                                            $begingroup$
                                            You can also prove this using group actions: $S_n$ acts on the set ${1, 2, dotsc, n}$, and the stabilizer of any $k$-set from that set is $S_ktimes S_{n-k}$. So by the orbit-stabilizer theorem, the number of such $k$-sets is the number of orbits, which is $frac{|S_n|}{|S_k||S_{n-k}|} = frac{n!}{k!(n-k)!}$.
                                            $endgroup$
                                            – rogerl
                                            Jul 27 '15 at 17:27




                                            $begingroup$
                                            You can also prove this using group actions: $S_n$ acts on the set ${1, 2, dotsc, n}$, and the stabilizer of any $k$-set from that set is $S_ktimes S_{n-k}$. So by the orbit-stabilizer theorem, the number of such $k$-sets is the number of orbits, which is $frac{|S_n|}{|S_k||S_{n-k}|} = frac{n!}{k!(n-k)!}$.
                                            $endgroup$
                                            – rogerl
                                            Jul 27 '15 at 17:27











                                            1












                                            $begingroup$

                                            Other examples of such many-proof theorems include:



                                            1) Harmonic series diverges



                                            There are at least 8 proofs of this fact based on different ideas.
                                            The first four of them can be found under this link: https://proofwiki.org/wiki/Harmonic_Series_is_Divergent



                                            The other four are of the following form:
                                            "If a subseries diverges, then the whole series diverges too. Now let's prove that the subseries of harmonic series, that formed only by reciprocals of primes diverges..."



                                            And there are at least four different ways to prove it: https://en.wikipedia.org/wiki/Divergence_of_the_sum_of_the_reciprocals_of_the_primes



                                            2) Variance of a random variable with support in $[0; 1]$ does not exceed $frac{1}{4}$



                                            There are at least three proofs of this, based on different ideas: What is the largest possible variance of a random variable on $[0; 1]$?



                                            3) No group is both a direct product and a direct product of non-trivial groups



                                            There are at least three proofs of this, based on different ideas: Does there exist a group that is both a free product and a direct product of nontrivial groups?



                                            4) $(mathbb{Q}, +)$ is not a direct product of any two non-trivial groups



                                            There are at least three proofs of this, based on different ideas: Is $(mathbb{Q},+)$ the direct product of two non-trivial subgroups?






                                            share|cite|improve this answer











                                            $endgroup$


















                                              1












                                              $begingroup$

                                              Other examples of such many-proof theorems include:



                                              1) Harmonic series diverges



                                              There are at least 8 proofs of this fact based on different ideas.
                                              The first four of them can be found under this link: https://proofwiki.org/wiki/Harmonic_Series_is_Divergent



                                              The other four are of the following form:
                                              "If a subseries diverges, then the whole series diverges too. Now let's prove that the subseries of harmonic series, that formed only by reciprocals of primes diverges..."



                                              And there are at least four different ways to prove it: https://en.wikipedia.org/wiki/Divergence_of_the_sum_of_the_reciprocals_of_the_primes



                                              2) Variance of a random variable with support in $[0; 1]$ does not exceed $frac{1}{4}$



                                              There are at least three proofs of this, based on different ideas: What is the largest possible variance of a random variable on $[0; 1]$?



                                              3) No group is both a direct product and a direct product of non-trivial groups



                                              There are at least three proofs of this, based on different ideas: Does there exist a group that is both a free product and a direct product of nontrivial groups?



                                              4) $(mathbb{Q}, +)$ is not a direct product of any two non-trivial groups



                                              There are at least three proofs of this, based on different ideas: Is $(mathbb{Q},+)$ the direct product of two non-trivial subgroups?






                                              share|cite|improve this answer











                                              $endgroup$
















                                                1












                                                1








                                                1





                                                $begingroup$

                                                Other examples of such many-proof theorems include:



                                                1) Harmonic series diverges



                                                There are at least 8 proofs of this fact based on different ideas.
                                                The first four of them can be found under this link: https://proofwiki.org/wiki/Harmonic_Series_is_Divergent



                                                The other four are of the following form:
                                                "If a subseries diverges, then the whole series diverges too. Now let's prove that the subseries of harmonic series, that formed only by reciprocals of primes diverges..."



                                                And there are at least four different ways to prove it: https://en.wikipedia.org/wiki/Divergence_of_the_sum_of_the_reciprocals_of_the_primes



                                                2) Variance of a random variable with support in $[0; 1]$ does not exceed $frac{1}{4}$



                                                There are at least three proofs of this, based on different ideas: What is the largest possible variance of a random variable on $[0; 1]$?



                                                3) No group is both a direct product and a direct product of non-trivial groups



                                                There are at least three proofs of this, based on different ideas: Does there exist a group that is both a free product and a direct product of nontrivial groups?



                                                4) $(mathbb{Q}, +)$ is not a direct product of any two non-trivial groups



                                                There are at least three proofs of this, based on different ideas: Is $(mathbb{Q},+)$ the direct product of two non-trivial subgroups?






                                                share|cite|improve this answer











                                                $endgroup$



                                                Other examples of such many-proof theorems include:



                                                1) Harmonic series diverges



                                                There are at least 8 proofs of this fact based on different ideas.
                                                The first four of them can be found under this link: https://proofwiki.org/wiki/Harmonic_Series_is_Divergent



                                                The other four are of the following form:
                                                "If a subseries diverges, then the whole series diverges too. Now let's prove that the subseries of harmonic series, that formed only by reciprocals of primes diverges..."



                                                And there are at least four different ways to prove it: https://en.wikipedia.org/wiki/Divergence_of_the_sum_of_the_reciprocals_of_the_primes



                                                2) Variance of a random variable with support in $[0; 1]$ does not exceed $frac{1}{4}$



                                                There are at least three proofs of this, based on different ideas: What is the largest possible variance of a random variable on $[0; 1]$?



                                                3) No group is both a direct product and a direct product of non-trivial groups



                                                There are at least three proofs of this, based on different ideas: Does there exist a group that is both a free product and a direct product of nontrivial groups?



                                                4) $(mathbb{Q}, +)$ is not a direct product of any two non-trivial groups



                                                There are at least three proofs of this, based on different ideas: Is $(mathbb{Q},+)$ the direct product of two non-trivial subgroups?







                                                share|cite|improve this answer














                                                share|cite|improve this answer



                                                share|cite|improve this answer








                                                edited Mar 24 at 11:36

























                                                answered Feb 7 at 19:32









                                                Yanior WegYanior Weg

                                                2,96321549




                                                2,96321549






























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