Finding the sum of 3 numbers with 5 positive integer divisors. The 2019 Stack Overflow...

RequirePermission not working

How to notate time signature switching consistently every measure

Keeping a retro style to sci-fi spaceships?

Why are there uneven bright areas in this photo of black hole?

Deal with toxic manager when you can't quit

Worn-tile Scrabble

How do you keep chess fun when your opponent constantly beats you?

How did passengers keep warm on sail ships?

For what reasons would an animal species NOT cross a *horizontal* land bridge?

Is it ethical to upload a automatically generated paper to a non peer-reviewed site as part of a larger research?

How can I have a shield and a way of attacking with a ranged weapon at the same time?

The difference between dialogue marks

Can there be female White Walkers?

How can I define good in a religion that claims no moral authority?

Is an up-to-date browser secure on an out-of-date OS?

Why isn't the circumferential light around the M87 black hole's event horizon symmetric?

What do I do when my TA workload is more than expected?

How to charge AirPods to keep battery healthy?

A word that means fill it to the required quantity

What could be the right powersource for 15 seconds lifespan disposable giant chainsaw?

Why “相同意思的词” is called “同义词” instead of "同意词"?

Is Cinnamon a desktop environment or a window manager? (Or both?)

Will it cause any balance problems to have PCs level up and gain the benefits of a long rest mid-fight?

How can I add encounters in the Lost Mine of Phandelver campaign without giving PCs too much XP?



Finding the sum of 3 numbers with 5 positive integer divisors.



The 2019 Stack Overflow Developer Survey Results Are InNumber theory with positive integer $n$ questionUpper bound for the difference between number-of-divisors and sum-of-divisors functionsSmallest positive integer with 60 divisorsFind the least positive integer with $24$ positive divisors.Counting and bounding square-free numbers formed from only the first $j$ primesThe product of five consecutive positive integers cannot be the square of an integerA conjecture concerning the number of divisors and the sum of divisors.Range of the map “product of positive divisors”Which natural numbers are the sum of three positive perfect squares?About The Sum of Positive Divisors of $n$












1












$begingroup$



What is the sum of the three numbers less that 1000 that have exactly
five positive integer divisors?




So I see that first since there are only 5 multiples, the number must be a perfect square. Then, I took a look at a few examples.



$#1text{:}$



$$2^2=4$$



$$impliestext{1, 4}$$



$$impliestext{2, 2}$$



4 then has 3 positive integer divisors.



$#2text{:}$



$$4^2=16$$



$$impliestext{1, 16}$$



$$impliestext{2, 8}$$



$$impliestext{4, 4}$$



This means that the square of a square number has 5 positive integer divisors. There are many of these under 1000, so how come there are three? I am probably missing a point here. What is wrong with my logic? How can I solve this problem?










share|cite|improve this question









$endgroup$












  • $begingroup$
    Your last conclusion is not right. For instance, $256$ is a square of a square number, but has many more than five positive integer divisors. Hint: prime number factorization.
    $endgroup$
    – Brian Tung
    Mar 22 at 0:12










  • $begingroup$
    Incidentally, there aren't that many fourth powers less than $1000$, but there are more than three. So you do need to narrow it down.
    $endgroup$
    – Brian Tung
    Mar 22 at 0:14
















1












$begingroup$



What is the sum of the three numbers less that 1000 that have exactly
five positive integer divisors?




So I see that first since there are only 5 multiples, the number must be a perfect square. Then, I took a look at a few examples.



$#1text{:}$



$$2^2=4$$



$$impliestext{1, 4}$$



$$impliestext{2, 2}$$



4 then has 3 positive integer divisors.



$#2text{:}$



$$4^2=16$$



$$impliestext{1, 16}$$



$$impliestext{2, 8}$$



$$impliestext{4, 4}$$



This means that the square of a square number has 5 positive integer divisors. There are many of these under 1000, so how come there are three? I am probably missing a point here. What is wrong with my logic? How can I solve this problem?










share|cite|improve this question









$endgroup$












  • $begingroup$
    Your last conclusion is not right. For instance, $256$ is a square of a square number, but has many more than five positive integer divisors. Hint: prime number factorization.
    $endgroup$
    – Brian Tung
    Mar 22 at 0:12










  • $begingroup$
    Incidentally, there aren't that many fourth powers less than $1000$, but there are more than three. So you do need to narrow it down.
    $endgroup$
    – Brian Tung
    Mar 22 at 0:14














1












1








1





$begingroup$



What is the sum of the three numbers less that 1000 that have exactly
five positive integer divisors?




So I see that first since there are only 5 multiples, the number must be a perfect square. Then, I took a look at a few examples.



$#1text{:}$



$$2^2=4$$



$$impliestext{1, 4}$$



$$impliestext{2, 2}$$



4 then has 3 positive integer divisors.



$#2text{:}$



$$4^2=16$$



$$impliestext{1, 16}$$



$$impliestext{2, 8}$$



$$impliestext{4, 4}$$



This means that the square of a square number has 5 positive integer divisors. There are many of these under 1000, so how come there are three? I am probably missing a point here. What is wrong with my logic? How can I solve this problem?










share|cite|improve this question









$endgroup$





What is the sum of the three numbers less that 1000 that have exactly
five positive integer divisors?




So I see that first since there are only 5 multiples, the number must be a perfect square. Then, I took a look at a few examples.



$#1text{:}$



$$2^2=4$$



$$impliestext{1, 4}$$



$$impliestext{2, 2}$$



4 then has 3 positive integer divisors.



$#2text{:}$



$$4^2=16$$



$$impliestext{1, 16}$$



$$impliestext{2, 8}$$



$$impliestext{4, 4}$$



This means that the square of a square number has 5 positive integer divisors. There are many of these under 1000, so how come there are three? I am probably missing a point here. What is wrong with my logic? How can I solve this problem?







number-theory






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked Mar 22 at 0:08









Max0815Max0815

81418




81418












  • $begingroup$
    Your last conclusion is not right. For instance, $256$ is a square of a square number, but has many more than five positive integer divisors. Hint: prime number factorization.
    $endgroup$
    – Brian Tung
    Mar 22 at 0:12










  • $begingroup$
    Incidentally, there aren't that many fourth powers less than $1000$, but there are more than three. So you do need to narrow it down.
    $endgroup$
    – Brian Tung
    Mar 22 at 0:14


















  • $begingroup$
    Your last conclusion is not right. For instance, $256$ is a square of a square number, but has many more than five positive integer divisors. Hint: prime number factorization.
    $endgroup$
    – Brian Tung
    Mar 22 at 0:12










  • $begingroup$
    Incidentally, there aren't that many fourth powers less than $1000$, but there are more than three. So you do need to narrow it down.
    $endgroup$
    – Brian Tung
    Mar 22 at 0:14
















$begingroup$
Your last conclusion is not right. For instance, $256$ is a square of a square number, but has many more than five positive integer divisors. Hint: prime number factorization.
$endgroup$
– Brian Tung
Mar 22 at 0:12




$begingroup$
Your last conclusion is not right. For instance, $256$ is a square of a square number, but has many more than five positive integer divisors. Hint: prime number factorization.
$endgroup$
– Brian Tung
Mar 22 at 0:12












$begingroup$
Incidentally, there aren't that many fourth powers less than $1000$, but there are more than three. So you do need to narrow it down.
$endgroup$
– Brian Tung
Mar 22 at 0:14




$begingroup$
Incidentally, there aren't that many fourth powers less than $1000$, but there are more than three. So you do need to narrow it down.
$endgroup$
– Brian Tung
Mar 22 at 0:14










2 Answers
2






active

oldest

votes


















2












$begingroup$

Recall: The number of divisors of a number $n$ whose prime decomposition is



$$n=p_1^{alpha_1}cdots p_k^{alpha_k}$$



Is $(alpha_1+1)cdots(alpha_k+1)$ (Exercise: prove this statement), then, you want this number to be $5$ and each factor being positive you must have all to be one except for one of them that must be $5$.



$5$ is prime you can only express it as $1cdot 5$; so if every factor is one, that means the exponent of that prime is $0$ in the factorization of $n$, and the factor that is $5$ tells us there's only one prime in the factorization and that the power is $5-1=4$ (solving for $alpha_text{something}$).



With a little algebra you get that $n=p^4$ for some prime number $p$, then, the only prime numbers whose fourth power is less than $1000$ are $p=2,3,5$ so the sum of the four numbers is



$$2^4+3^4+5^4=722$$






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Can you explain how you got to that $n=p^4$?
    $endgroup$
    – Max0815
    Mar 22 at 0:54










  • $begingroup$
    @Max0815 Well, since $5$ is prime you can only express it as $1cdot5$; so if every factor is one, that means the exponent of that prime is $0$ in the factorization of $n$, and the factor that is $5$ tells us there's only one prime in the factorization and that the power is $5-1=4$ (solving for $alpha_{something}$)
    $endgroup$
    – Bruno Andrades
    Mar 22 at 0:58










  • $begingroup$
    thank you I see how you got that!
    $endgroup$
    – Max0815
    Mar 22 at 0:59










  • $begingroup$
    @Max0815 Glad to help! :)
    $endgroup$
    – Bruno Andrades
    Mar 22 at 0:59










  • $begingroup$
    I added the clarification part to the answer. Its useful to me. After the edit is accepeted, I'll accept your answer :)
    $endgroup$
    – Max0815
    Mar 22 at 1:02



















1












$begingroup$

The three numbers less than 1000 with exactly 5 factors are:




  • 16 {1, 2, 4, 8, 16}

  • 81 {1, 3, 9, 27, 81}

  • 625 {1, 5, 25, 125, 625}


So the sum is: 722, just as Max wrote.






share|cite|improve this answer









$endgroup$














    Your Answer





    StackExchange.ifUsing("editor", function () {
    return StackExchange.using("mathjaxEditing", function () {
    StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
    StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
    });
    });
    }, "mathjax-editing");

    StackExchange.ready(function() {
    var channelOptions = {
    tags: "".split(" "),
    id: "69"
    };
    initTagRenderer("".split(" "), "".split(" "), channelOptions);

    StackExchange.using("externalEditor", function() {
    // Have to fire editor after snippets, if snippets enabled
    if (StackExchange.settings.snippets.snippetsEnabled) {
    StackExchange.using("snippets", function() {
    createEditor();
    });
    }
    else {
    createEditor();
    }
    });

    function createEditor() {
    StackExchange.prepareEditor({
    heartbeatType: 'answer',
    autoActivateHeartbeat: false,
    convertImagesToLinks: true,
    noModals: true,
    showLowRepImageUploadWarning: true,
    reputationToPostImages: 10,
    bindNavPrevention: true,
    postfix: "",
    imageUploader: {
    brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
    contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
    allowUrls: true
    },
    noCode: true, onDemand: true,
    discardSelector: ".discard-answer"
    ,immediatelyShowMarkdownHelp:true
    });


    }
    });














    draft saved

    draft discarded


















    StackExchange.ready(
    function () {
    StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3157564%2ffinding-the-sum-of-3-numbers-with-5-positive-integer-divisors%23new-answer', 'question_page');
    }
    );

    Post as a guest















    Required, but never shown

























    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    2












    $begingroup$

    Recall: The number of divisors of a number $n$ whose prime decomposition is



    $$n=p_1^{alpha_1}cdots p_k^{alpha_k}$$



    Is $(alpha_1+1)cdots(alpha_k+1)$ (Exercise: prove this statement), then, you want this number to be $5$ and each factor being positive you must have all to be one except for one of them that must be $5$.



    $5$ is prime you can only express it as $1cdot 5$; so if every factor is one, that means the exponent of that prime is $0$ in the factorization of $n$, and the factor that is $5$ tells us there's only one prime in the factorization and that the power is $5-1=4$ (solving for $alpha_text{something}$).



    With a little algebra you get that $n=p^4$ for some prime number $p$, then, the only prime numbers whose fourth power is less than $1000$ are $p=2,3,5$ so the sum of the four numbers is



    $$2^4+3^4+5^4=722$$






    share|cite|improve this answer











    $endgroup$













    • $begingroup$
      Can you explain how you got to that $n=p^4$?
      $endgroup$
      – Max0815
      Mar 22 at 0:54










    • $begingroup$
      @Max0815 Well, since $5$ is prime you can only express it as $1cdot5$; so if every factor is one, that means the exponent of that prime is $0$ in the factorization of $n$, and the factor that is $5$ tells us there's only one prime in the factorization and that the power is $5-1=4$ (solving for $alpha_{something}$)
      $endgroup$
      – Bruno Andrades
      Mar 22 at 0:58










    • $begingroup$
      thank you I see how you got that!
      $endgroup$
      – Max0815
      Mar 22 at 0:59










    • $begingroup$
      @Max0815 Glad to help! :)
      $endgroup$
      – Bruno Andrades
      Mar 22 at 0:59










    • $begingroup$
      I added the clarification part to the answer. Its useful to me. After the edit is accepeted, I'll accept your answer :)
      $endgroup$
      – Max0815
      Mar 22 at 1:02
















    2












    $begingroup$

    Recall: The number of divisors of a number $n$ whose prime decomposition is



    $$n=p_1^{alpha_1}cdots p_k^{alpha_k}$$



    Is $(alpha_1+1)cdots(alpha_k+1)$ (Exercise: prove this statement), then, you want this number to be $5$ and each factor being positive you must have all to be one except for one of them that must be $5$.



    $5$ is prime you can only express it as $1cdot 5$; so if every factor is one, that means the exponent of that prime is $0$ in the factorization of $n$, and the factor that is $5$ tells us there's only one prime in the factorization and that the power is $5-1=4$ (solving for $alpha_text{something}$).



    With a little algebra you get that $n=p^4$ for some prime number $p$, then, the only prime numbers whose fourth power is less than $1000$ are $p=2,3,5$ so the sum of the four numbers is



    $$2^4+3^4+5^4=722$$






    share|cite|improve this answer











    $endgroup$













    • $begingroup$
      Can you explain how you got to that $n=p^4$?
      $endgroup$
      – Max0815
      Mar 22 at 0:54










    • $begingroup$
      @Max0815 Well, since $5$ is prime you can only express it as $1cdot5$; so if every factor is one, that means the exponent of that prime is $0$ in the factorization of $n$, and the factor that is $5$ tells us there's only one prime in the factorization and that the power is $5-1=4$ (solving for $alpha_{something}$)
      $endgroup$
      – Bruno Andrades
      Mar 22 at 0:58










    • $begingroup$
      thank you I see how you got that!
      $endgroup$
      – Max0815
      Mar 22 at 0:59










    • $begingroup$
      @Max0815 Glad to help! :)
      $endgroup$
      – Bruno Andrades
      Mar 22 at 0:59










    • $begingroup$
      I added the clarification part to the answer. Its useful to me. After the edit is accepeted, I'll accept your answer :)
      $endgroup$
      – Max0815
      Mar 22 at 1:02














    2












    2








    2





    $begingroup$

    Recall: The number of divisors of a number $n$ whose prime decomposition is



    $$n=p_1^{alpha_1}cdots p_k^{alpha_k}$$



    Is $(alpha_1+1)cdots(alpha_k+1)$ (Exercise: prove this statement), then, you want this number to be $5$ and each factor being positive you must have all to be one except for one of them that must be $5$.



    $5$ is prime you can only express it as $1cdot 5$; so if every factor is one, that means the exponent of that prime is $0$ in the factorization of $n$, and the factor that is $5$ tells us there's only one prime in the factorization and that the power is $5-1=4$ (solving for $alpha_text{something}$).



    With a little algebra you get that $n=p^4$ for some prime number $p$, then, the only prime numbers whose fourth power is less than $1000$ are $p=2,3,5$ so the sum of the four numbers is



    $$2^4+3^4+5^4=722$$






    share|cite|improve this answer











    $endgroup$



    Recall: The number of divisors of a number $n$ whose prime decomposition is



    $$n=p_1^{alpha_1}cdots p_k^{alpha_k}$$



    Is $(alpha_1+1)cdots(alpha_k+1)$ (Exercise: prove this statement), then, you want this number to be $5$ and each factor being positive you must have all to be one except for one of them that must be $5$.



    $5$ is prime you can only express it as $1cdot 5$; so if every factor is one, that means the exponent of that prime is $0$ in the factorization of $n$, and the factor that is $5$ tells us there's only one prime in the factorization and that the power is $5-1=4$ (solving for $alpha_text{something}$).



    With a little algebra you get that $n=p^4$ for some prime number $p$, then, the only prime numbers whose fourth power is less than $1000$ are $p=2,3,5$ so the sum of the four numbers is



    $$2^4+3^4+5^4=722$$







    share|cite|improve this answer














    share|cite|improve this answer



    share|cite|improve this answer








    edited Mar 22 at 1:39









    Max0815

    81418




    81418










    answered Mar 22 at 0:24









    Bruno AndradesBruno Andrades

    24811




    24811












    • $begingroup$
      Can you explain how you got to that $n=p^4$?
      $endgroup$
      – Max0815
      Mar 22 at 0:54










    • $begingroup$
      @Max0815 Well, since $5$ is prime you can only express it as $1cdot5$; so if every factor is one, that means the exponent of that prime is $0$ in the factorization of $n$, and the factor that is $5$ tells us there's only one prime in the factorization and that the power is $5-1=4$ (solving for $alpha_{something}$)
      $endgroup$
      – Bruno Andrades
      Mar 22 at 0:58










    • $begingroup$
      thank you I see how you got that!
      $endgroup$
      – Max0815
      Mar 22 at 0:59










    • $begingroup$
      @Max0815 Glad to help! :)
      $endgroup$
      – Bruno Andrades
      Mar 22 at 0:59










    • $begingroup$
      I added the clarification part to the answer. Its useful to me. After the edit is accepeted, I'll accept your answer :)
      $endgroup$
      – Max0815
      Mar 22 at 1:02


















    • $begingroup$
      Can you explain how you got to that $n=p^4$?
      $endgroup$
      – Max0815
      Mar 22 at 0:54










    • $begingroup$
      @Max0815 Well, since $5$ is prime you can only express it as $1cdot5$; so if every factor is one, that means the exponent of that prime is $0$ in the factorization of $n$, and the factor that is $5$ tells us there's only one prime in the factorization and that the power is $5-1=4$ (solving for $alpha_{something}$)
      $endgroup$
      – Bruno Andrades
      Mar 22 at 0:58










    • $begingroup$
      thank you I see how you got that!
      $endgroup$
      – Max0815
      Mar 22 at 0:59










    • $begingroup$
      @Max0815 Glad to help! :)
      $endgroup$
      – Bruno Andrades
      Mar 22 at 0:59










    • $begingroup$
      I added the clarification part to the answer. Its useful to me. After the edit is accepeted, I'll accept your answer :)
      $endgroup$
      – Max0815
      Mar 22 at 1:02
















    $begingroup$
    Can you explain how you got to that $n=p^4$?
    $endgroup$
    – Max0815
    Mar 22 at 0:54




    $begingroup$
    Can you explain how you got to that $n=p^4$?
    $endgroup$
    – Max0815
    Mar 22 at 0:54












    $begingroup$
    @Max0815 Well, since $5$ is prime you can only express it as $1cdot5$; so if every factor is one, that means the exponent of that prime is $0$ in the factorization of $n$, and the factor that is $5$ tells us there's only one prime in the factorization and that the power is $5-1=4$ (solving for $alpha_{something}$)
    $endgroup$
    – Bruno Andrades
    Mar 22 at 0:58




    $begingroup$
    @Max0815 Well, since $5$ is prime you can only express it as $1cdot5$; so if every factor is one, that means the exponent of that prime is $0$ in the factorization of $n$, and the factor that is $5$ tells us there's only one prime in the factorization and that the power is $5-1=4$ (solving for $alpha_{something}$)
    $endgroup$
    – Bruno Andrades
    Mar 22 at 0:58












    $begingroup$
    thank you I see how you got that!
    $endgroup$
    – Max0815
    Mar 22 at 0:59




    $begingroup$
    thank you I see how you got that!
    $endgroup$
    – Max0815
    Mar 22 at 0:59












    $begingroup$
    @Max0815 Glad to help! :)
    $endgroup$
    – Bruno Andrades
    Mar 22 at 0:59




    $begingroup$
    @Max0815 Glad to help! :)
    $endgroup$
    – Bruno Andrades
    Mar 22 at 0:59












    $begingroup$
    I added the clarification part to the answer. Its useful to me. After the edit is accepeted, I'll accept your answer :)
    $endgroup$
    – Max0815
    Mar 22 at 1:02




    $begingroup$
    I added the clarification part to the answer. Its useful to me. After the edit is accepeted, I'll accept your answer :)
    $endgroup$
    – Max0815
    Mar 22 at 1:02











    1












    $begingroup$

    The three numbers less than 1000 with exactly 5 factors are:




    • 16 {1, 2, 4, 8, 16}

    • 81 {1, 3, 9, 27, 81}

    • 625 {1, 5, 25, 125, 625}


    So the sum is: 722, just as Max wrote.






    share|cite|improve this answer









    $endgroup$


















      1












      $begingroup$

      The three numbers less than 1000 with exactly 5 factors are:




      • 16 {1, 2, 4, 8, 16}

      • 81 {1, 3, 9, 27, 81}

      • 625 {1, 5, 25, 125, 625}


      So the sum is: 722, just as Max wrote.






      share|cite|improve this answer









      $endgroup$
















        1












        1








        1





        $begingroup$

        The three numbers less than 1000 with exactly 5 factors are:




        • 16 {1, 2, 4, 8, 16}

        • 81 {1, 3, 9, 27, 81}

        • 625 {1, 5, 25, 125, 625}


        So the sum is: 722, just as Max wrote.






        share|cite|improve this answer









        $endgroup$



        The three numbers less than 1000 with exactly 5 factors are:




        • 16 {1, 2, 4, 8, 16}

        • 81 {1, 3, 9, 27, 81}

        • 625 {1, 5, 25, 125, 625}


        So the sum is: 722, just as Max wrote.







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Mar 22 at 1:47









        David G. StorkDavid G. Stork

        12.1k41836




        12.1k41836






























            draft saved

            draft discarded




















































            Thanks for contributing an answer to Mathematics Stack Exchange!


            • Please be sure to answer the question. Provide details and share your research!

            But avoid



            • Asking for help, clarification, or responding to other answers.

            • Making statements based on opinion; back them up with references or personal experience.


            Use MathJax to format equations. MathJax reference.


            To learn more, see our tips on writing great answers.




            draft saved


            draft discarded














            StackExchange.ready(
            function () {
            StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3157564%2ffinding-the-sum-of-3-numbers-with-5-positive-integer-divisors%23new-answer', 'question_page');
            }
            );

            Post as a guest















            Required, but never shown





















































            Required, but never shown














            Required, but never shown












            Required, but never shown







            Required, but never shown

































            Required, but never shown














            Required, but never shown












            Required, but never shown







            Required, but never shown







            Popular posts from this blog

            Nidaros erkebispedøme

            Birsay

            Was Woodrow Wilson really a Liberal?Was World War I a war of liberals against authoritarians?Founding Fathers...