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 4n + 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.