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Let $p$ be a prime number. Let $x,y,a,b$ be distinct positive integers. If $p=x^2 + y^2$ then $p\ne a^2+b^2$. Is this true ? If so why ? What are the proofs for this ? I know the Fibonacci identitity : , but I do not see how it follows.

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up vote 2 down vote accepted

This is the basis of Eulers factorization method you can find full proof enclosed. (Brahmagupta-Fibonacci is an important part of it)

Given two different sums of square representations of a number, this produces a factorization. Euler used this to factor huge mersenne numbers and break the world record for biggest prime.

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If people were interested in the relation between the two answers: This one is a constructive proof in that it actually gives you a factorization and it is elementary in the sense of not using complex numbers. The proof with Z[i] only gives existence of a factorization. – user58512 Jan 28 '13 at 13:03
+1 At the moment I accepted your answer. It was better then the other from the start. However after the edit the other answer seems improved. See my comment there. – mick Jan 28 '13 at 22:34

This is a consequence that $\mathbb{Z}[i]$ is a UFD; since $p$ can be factored as $(x+yi)(x-yi)$, and both $x+yi$ and $x-yi$ are clearly both primes, then by unique factorization $p$ cannot be factored in any other way (up to order and unit).

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I do not consider this as an answer. Primes can not be factored is clearly true. – mick Jan 27 '13 at 22:49
@mick Do you understand what $\mathbb Z[i]$ is, and what a UFD is? – Andrés Caicedo Jan 27 '13 at 22:54
@mick well, this does answer your question. What do you mean when you write "Primes can not be factored is clearly true"? – Olivier Bégassat Jan 27 '13 at 22:56
@mick: $p$ is a prime in $\mathbb Z$, but is not in $\mathbb Z[i]$ as a factorization has been displayed. – Ross Millikan Jan 28 '13 at 1:03
@mick The point is that in $\mathbb{Z}[i]$, the number $p=x^2+y^2$ is no longer prime; it has (non-unit) factors. On the other hand, the number $x+yi$ is prime; it has no non-unit factors. – Steven Stadnicki Jan 28 '13 at 1:04

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