Prime Numbers

[Pandaren]

Hey everyone, I need help on this problem:

Find all prime numbers x such that x^2 = v^3 + 1 for some integer v.

Thanks a lot for your help, i appreciated.

[Hurkyl]

Try factoring.

[Pandaren]

After I factored the right side, i got x^2 = (v + 1)(v^2 - v + 1) I dont see what to do next :frown:

Thank you for your help :smile:

[Hurkyl]

You've found 2 factors, in terms of v.
Can you find all factors, in terms of x?

[Pandaren]

I dont understand the "all factors of x" part
you mean (x - 1)(x + 1) = v^3 ?

[Pandaren]

The only x that seem to work is x = 2

[Hurkyl]

Well, x^2 is the square of the prime, x, right? You know the prime factorization of x^2, so you can write down all of its factors.


Since v+1 is a factor, v+1 must be equal to one of the numbers on that list...

[Pandaren]

Well, x^2 is the square of the prime, x, right? You know the prime factorization of x^2, so you can write down all of its factors.


Since v+1 is a factor, v+1 must be equal to one of the numbers on that list...
you mean if 2 is a prime, then 2 = x, 2^2 = x^2
Then 2^2 = (v + 1)(v^2 - v + 1) Or 2 * 2 = (v + 1)(v^2 - v + 1)
Therefore 2 = v + 1 and 2 = v^2 - v + 1?

[Hurkyl]

You know that (v+1)(v^2-v+1) is a factorization of x^2 into two factors, and you know that x * x is a factorization of x^2 into two factors, but you don't know that they're the same factorization.

However, you can write down all of the ways to factor x^2 into two factors, and you know that (v+1)(v^2-v+1) is going to be one of them.

[Pandaren]

Ohh I understand now thanks for your help