Hi, I'veI've been studying some proofs of Thethe four squares-square theorem. Some of them are pretty clear. However, I came across a statement that Thethe four squares-square theorem can be easily derived from Gauss-Legendre three squares-square theorem. Hard as I tried, I couldn't find out how to do it.
I was hoppinghoping someone could give me some idea or point out some article.
Thank you!