Finding the sum of 3 numbers with 5 positive integer divisors. The 2019 Stack Overflow...
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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 $n$
$begingroup$
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?
number-theory
$endgroup$
add a comment |
$begingroup$
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?
number-theory
$endgroup$
$begingroup$
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
add a comment |
$begingroup$
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?
number-theory
$endgroup$
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
number-theory
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
add a comment |
$begingroup$
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
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
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
$$2^4+3^4+5^4=722$$
$endgroup$
$begingroup$
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
add a comment |
$begingroup$
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.
$endgroup$
add a comment |
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
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
$$2^4+3^4+5^4=722$$
$endgroup$
$begingroup$
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
add a comment |
$begingroup$
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
$$2^4+3^4+5^4=722$$
$endgroup$
$begingroup$
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
add a comment |
$begingroup$
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
$$2^4+3^4+5^4=722$$
$endgroup$
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
$$2^4+3^4+5^4=722$$
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
add a comment |
$begingroup$
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
add a comment |
$begingroup$
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.
$endgroup$
add a comment |
$begingroup$
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.
$endgroup$
add a comment |
$begingroup$
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.
$endgroup$
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.
answered Mar 22 at 1:47
David G. StorkDavid G. Stork
12.1k41836
12.1k41836
add a comment |
add a comment |
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$begingroup$
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