In an email dated 29/5/2005 9:44:59 am GMT Daylight time, najiba amimar <

najibaamimar@...> writes:

>Every prime number of the form 6n+1

>can be written as x**2 + 3 * y**2 (Euler)

>and

>every prime number of the form 4n+1

>can be written as x**2 + y**2 (Fermat)

>These two theorems have been demonstated

>more than two century ago, but the proofs

>seems tremendously difficult to find on

>the web!

This is partly because the proofs are not that straightforward!

As so often, Hardy & Wright is a good source.

They give four distinct proofs of a more general theorem than the one you refer to:-

<<

Theorem 366. A number n is the sum of two square if and only if all prime factors of n of the form 4m+3 have even exponents n the standard form of n.

>>

Perhaps the simplest method is by using the properties of the Gaussian primes, viz. those of Z[i].

The analogous theorem for numbers of the form 3m+1 resp. 3m+2 can be found and proved by investigating the Eisenstein primes, viz. those of the ring Z[w], where w = -1/2+sqrt(-3)/2.

-Mike Oakes