is known the classical Pell equation. The Pell equation x2 − Dy2 = 1, was first
studied by Brahmagupta (598-670) and Bhaskara (1114-1185). Its complete
theory was worked out by Lagrange (1736-1813), not Pell. It is often said that
Euler (1707-1783) mistakenly attributed Brouncker’s (1620-1684) work on this
equation to Pell. However the equation appears in a book by Rahn (1622-1676)
which was certainly written with Pell’s help: some say entirely written by Pell.
Perhaps Euler knew what he was doing in naming the equation.