# Diophantine Equations

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## diophantine equations

[¦dī·ə¦fant·ən i′kwā·zhənz]*The Great Soviet Encyclopedia*(1979). It might be outdated or ideologically biased.

## Diophantine Equations

(named after the ancient Greek mathematician Diophantus), algebraic equations or systems of algebraic equations with integral coefficients, where the number of unknowns exceeds the number of equations and where we are looking for integral or rational solutions. The concept of Diophantine equations has been broadened in modern mathematics: they are equations in which the required solutions are algebraic numbers. Diophantine equations are also called indeterminate equations.

The simplest Diophantine equation is *ax* + *by* = 1, where *a* and *b* are relatively prime integers. This equation has an infinite number of solutions: if *x*_{0} and *y*_{0} are one solution, then the numbers *x* = *x*_{0}, + *bn* and *y* = *y*_{0} − *an* (*n* is any integer) will also be solutions. Thus, all integral solutions of the equation 2*x* + 3*y* = 1 are obtained from the formulas *x* = 2 + 3*n* and y = −1 − 2*n* (here, *x*_{0} = 2 and *y*_{0} = −1). Another example of a Diophantine equation is *x*^{2} + *y*^{2} = *z*^{2}. The positive integral solutions of this equation are the lengths of the legs *x* and *y* and the hypotenuse *z* of right triangles whose sides are integral and are called Pythagorean numbers. All triplets of relatively prime Pythagorean numbers can be obtained from the formulas *x* = *m*^{2} − *n*^{2}, *y* = 2*mn*, and *z* = *m*^{2} + *n ^{2}*, where

*m*and

*n*are integers (

*m*>

*n*> 0).

In his work *Arithmetica*, Diophantus attempted to find rational (not necessarily integral) solutions for special types of Diophantine equations. The general theory of Diophantine equations of the first degree was developed in the 17th century by the French mathematician C.-G. Bachet de Meziriac. By the early 19th century P. de Fermat, J. Wallis, L. Euler, J. Lagrange, and K. Gauss had investigated Diophantine equations of the type

*ax*^{2} + *bxy* + *cy*^{2} + *dx* + *ey* + *f* = 0

where *a, b, c, d, e*, and *f* are integers, that is, a general nonhomogeneous equation of the second degree in two unknowns. Fermat maintained, for example, that the Diophantine equation *x*^{2} − *dy*^{2} = 1 (the Pellian equation), where *d* is a positive integer that is not a perfect square, has an infinite number of solutions. Wallis and Euler provided methods for solving this equation, and Lagrange proved that the number of solutions was infinite. Using continued fractions, Lagrange studied the general nonhomogeneous Diophantine equation of the second degree in two unknowns. Gauss constructed a general theory of quadratic forms that serves as the basis for solving certain types of Diophantine equations. Only in the 20th century were important gains made in the study of Diophantine equations in two unknowns of degree higher than the second. A. Thue showed that the Diophantine equation

*a*_{0}*x ^{n}* +

*a*

_{1}

*x*

^{n−1}

*y*+ … +

*a*=

_{n}y^{n}*c*

(where *n ≥* 3; and *a*_{0}, *a*_{1}, …, *a _{n}* and

*c*are integers and the polynomial

*a*

_{0}

*t*+

^{n}*a*

_{1}

*t*

^{n−1}+ … +

*a*

_{n}is irreducible in the field of rational numbers) cannot have an infinite number of integral solutions. The British mathematician A. Baker obtained bounds on the solutions of some of these equations. The method of B. N. Delone applies to a narrower class of Diophantine equations but yields bounds on the number of solutions. In particular, his method completely solves Diophantine equations of the form

*ax*^{3} + *y*^{3} = 1

There are many directions in the theory of Diophantine equations. Thus, Fermat’s last theorem is a well-known problem in the theory of Diophantine equations. Soviet mathematicians (B. N. Delone, A. O. Gel’fond, and D. K. Faddeev, among others) have made fundamental contributions to the theory of Diophantine equations.

### REFERENCES

Gel’fond, A. O.*Reshenie uravnenii v tselykh chislakh*, 2nd ed. Moscow, 1956.

Dickson, L. E.

*History of the Theory of Numbers*, vol. 2. Washington, D.C., 1920.

Skolem, Th.

*Diophantische Gleichungen*. Berlin, 1938.