Because all the elements in A must have images, and the elements in B do not necessarily have original images, according to the principle of multiplication, it is of course N m.

Let elements a 1, a2, ... I am in A; Elements b 1, b2, ... Then element a 1 in A can be mapped to b 1, b2, ..., bn, * * n mappings; Similarly, a2 has n mappings; ……; Am also has n mappings, so a * * * is n * n * ... * n = n m.

Conditions for establishment:

A simple expression of the mapping conditions;

1, ergodicity of domain: each element X in X has a corresponding object in the mapped value domain.

2. Uniqueness of correspondence: one element in the definition domain can only correspond to one element in the mapping range. The definition domain is also called the original image set, and the value domain is also called the image set.