4. Pell's equation and continued fractions
Euler's interest in Tell's equation' x2 - Ny1 = 1 had been aroused in
1730 - as it is well known the name Pell was attached to the equation by him
under a misapprehension when he published a short paper on a more general
equation on the subject, including a description of Brouncker's method. He
was aware that the solutions supply good approximations to VÑ.
A real number a has the continued fraction expansion