A solution to a problem of Fermat, on two numbers of which the sum is a   square and the sum of their squares is a biquadrate, inspired by the   Illustrious La Grange
  Euler wants to find rational numbers (integers) x and y such that x+y is a square and x^2+y^2 is a fourth power. He parametrizes these with two other variables that satisfy certain equations. 
Leonhard Euler