Fermat's theorem on sums of two squares - Wikipedia
https://en.wikipedia.org/wiki/Fermat's_theorem_on_sums_of_two_squaresFermat usually did not write down proofs of his claims, and he did not provide a proof of this statement. The first proof was found by Euler after much effort and is based on infinite descent. He announced it in two letters to Goldbach, on May 6, 1747 and on April 12, 1749; he published the detailed proof in two articles (between 1752 and 1755). Lagrange gave a proof in 1775 that was based on his study of quadratic forms. This proof was simplified by Gauss in his Disquisitiones Ar…
FERMAT’S TWO SQUARES THEOREM
https://core.ac.uk/download/pdf/97080.pdfFERMAT’S TWO SQUARES THEOREM Pierre de Fermat (1601–1665) was a French lawyer. He has been described as the greatest amateur mathematician of all time, for his contributions to optics, probability, and, most notably, number theory. Perhaps he is best known for “Fermat’s Last Theorem” — the (still unproved) assertion that
Art of Problem Solving
artofproblemsolving.com › wiki › indexFermat's Two Squares Theorem states that that a prime number can be represented as a sum of two nonzero squares if and only if or ; and that this representation is unique. Fermat first listed this theorem in 1640, but listed it without proof, as was usual for him. Euler gave the first written proof in 1747, by infinite descent.