# 15th June

On this day in 1641, in a letter to Bernard Frenicle de Bessy, Fermat called the theorem that every prime of the form 4

*n*+ 1 is the sum of two squares, the*fundamental theorem of right triangles*. He stated that he had a proof that was "irrefutable". Later he suggested he had a proof by infinite descent. Euler is credited with the first correct proof of the theorem, still called Fermat's theorem.