DEFINITION 1 函数 Let a and b be sets. a function f from a to b is an assignment of exactly one element of b to each element ofA. We write f(a)=b if b is the unique element of B assigned by the function f to the element a ofA. If f is a function from A to B. we write f:A-BF u n c t i o n s 函 数 DEFINITION 1. Let A and B be sets. A function f from A to B is an assignment of exactly one element of B to each element of A. We write f(a) = b if b is the unique element of B assigned by the function f to the element a of A. If f is a function from A to B, we write f : A→B