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I need help with the following two generalities:

What is the characterization of primes that are sums of two squares?

And what is the characterization of $n\ge 1$ that are sums of two squares?

I'm hoping I'm over thinking these two generalities, but they seem extremely broad to me.

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Take a look here. –  P.. Nov 9 '12 at 14:57

1 Answer 1

up vote 6 down vote accepted

This is sometimes called Fermat's Christmas theorem. The primes are $2$ and all those of the form $4k+1$. For composites, all prime factors of the form $4k+3$ must occur an even number of times.

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