20th February

On this day in 1648, a letter from Pierre de Fermat through Frenicle de Bessy to Kenelm Digby reached John Wallis saying that Fermat had solved equations of the type x2Ay2=1x^2 - Ay^2 = 1 for all non-square values up to 150. Thus begins the saga of the mis-naming of Pell's equation.