Expressing Fibonacci numbers as the sum of squares.

Question

As we know that by observation, the Fibonacci numbers ($F_0=0$, $F_1=1$, $F_{n}=F_{n-1}+F_{n-2}$) have the identity $$F_{2k+1}=F_k^2 + F_{k+1}^2.$$ In particular, if $n$ is odd, then $F_n$ is a sum of two squares. Are there infinitely many even $n$ for which $F_n$ is a sum of two squares? If yes, how one can generalize by giving a proper proof or method?

Answer 1

See the answers at http://mathoverflow.net/questions/67601/which-fibonacci-numbers-are-the-sum-of-two-squares