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Prove that $n$ is a sum of two squares?
Fermat’s theorem on sums of two squares

Is there any elementary proof for this theorem: A number $n$ is a sum of two squares if and only if all prime factors of of the form $4k+3$ have even exponent in the prime factorization of $n$.

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    As far as I remember, this wasn't that easy, needing the quadratic residue theorem and Euklid's identity (I hope it's called this way).2012-11-28

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