Greek Number Theory

Number theory is the second large field of mathematics that comes to us from the Pythagoreans via Euclid. The Pythagorean theorem led mathematicians to the study of squares and sums of squares; Euclid drew attention to the

primes

by proving that there are infinitely many of them. Euclid’s investigations were based on the so-called

Euclidean algorithm

, a method for finding the greatest common divisor of two natural numbers. Common divisors are the key to basic results about prime numbers, in particular

unique prime factorization

, which says that each natural number factors into primes in exactly one way. Another discovery of the Pythagoreans, the irrationality of

$$\sqrt{2}$$

, has repercussions in the world of natural numbers. Since

$$\sqrt{2}\neq m/n$$

for any natural numbers

m

,

n

, there is no solution of the equation

$$x^2 - 2y^2 = 0$$

in the natural numbers. But, surprisingly, there are natural number solutions of

$$x^2 - \rm{2}y^2 = 1$$

, and in fact infinitely many of them. The same is true of the equation

$$x^2 - Ny^2 = 1$$

for any nonsquare natural number

N

. The latter equation, called

Pell’s equation

, is perhaps second in fame only to the Pythagorean equation

$$x^2 + y^2 = z^2$$

, among equations for which integer solutions are sought. Methods for solving the Pell equation for general

N

were first discovered by Indian mathematicians, whose work we study in Chapter 5. Equations for which integer or rational solutions are sought are called

Diophantine

, after Diophantus. The methods he used to solve quadratic and cubic Diophantine equations are still of interest. We study his method for cubics in this chapter, and take it up again in Chapters 11 and 16.