Number is the basis of modern mathematics.But what is number? What does it mean to say that多+多=1，.}=，and(-1)(-l)=1? We learn in school the mechanics of handling fractions and negative numbers,but for a real understanding of the number system we must go back to simpler elements.While the Greeks chose the geometrical con- cepts of point and line as the basis of their mathematics,it has become the modern guiding principle that all mathematical statements should be reducible ultiinately to statements about the nalural numbers, 1,2,3,...."God created the natural numbers;everything else is man's handiwork."In these words Leopold Kronecker (1823-1891) pointed out the safe ground on which the structure of mathematics can be built. Fortunately,the mathematician as such need not be concerned with the philosophical nature of the transition from collections of concrete objects to the abstract number concept.We shall therefore accept the natural numbers as given,togcther with the two fundamental opera- tions,addition and multiplication,by which they may be combined. from What is Mathematics,by Courant and Robbinsfrom What is Mathematics, by Courant and Robbins