# Pell's Equation

(redirected from*Fermat's equation*)

The following article is from

*The Great Soviet Encyclopedia*(1979). It might be outdated or ideologically biased.## Pell’s Equation

an equation of the form *x*^{2} - *Dy*^{2} = 1, where *D* is a positive integer that is not a perfect square and the equation is to be solved in integers. The equation has an infinite number of solutions. The solution *x*_{0} = 1, *y*_{0} = 0 is obvious. The next—in terms of magnitude—solution (*x*_{1}, *y*_{1}) of Pell’s equation can be found by expanding into a continued fraction. If we know the solution (*x*_{1}, *y*_{1}), the entire set of solutions (*x _{n} y_{n}*) can be obtained by using the formula

Pell’s equation is closely related to the theory of algebraic numbers. It is named after the 17th-century British mathematician J. Pell, to whom L. Euler incorrectly attributed a method of solving this equation.

### REFERENCES

Venkov, B. A.*Elementarnaia teoriia chisel*. Moscow-Leningrad, 1937. Chapter 2.

Dickson, L. E.

*History of the Theory of Numbers*, vol. 2. New York, 1966.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.