关于一直弄混的三个定义injective, surjective

从网站上搜寻到的比较好的解释

网址为 http://www.mathsisfun.com/sets/injective-surjective-bijective.html

Function: No element in A is left.

                   One-to-many is not OK.

Injective:

Many-to-one is not OK.

Subjective:

No element in B is left.

Bijective:

One-to-one perfect correspondence.