I am wondering if anyone knows where I can find a proof for the statement "any non-negative integer can be identified as the sum of four squares" i.e., x^2 = a^2 + b^2 + c^2 + d^2. I believe it was solved by Andrew Wyle, related to his proof of Fermat's Last Theorem. Thanks a lot for any help!!

-Joe

This is a well known theorem that has been proven by Lagrange in 1770. It's variably known as the 4 Squares Theorem, Lagrange's Theorem, and Bachet's Theorem. For proofs see

1. G.H. Hardy, E.M. Wright, An Introduction to the Theory of Numbers, Oxford University Press, 1979, p. 302 (available at amazon.com)
   
2. T. Nagell, Introduction to Number Theory, Chelsea Publishing, 1981, p. 191 (available at https://www.ams.org/bookstore?fn=20&arg1=chelsealist&item=CHEL-163|ams.org>)