If it exists,

Female:

a-& gt；

b，

but

F is a one-to-one function and f is a mapping function.

Then it's called F.

This is a,

B

A one-to-one correspondence between

In some cases,

You can put

a

and

f(a)

Thought it was the same,

And will

One,

B

Treat as the same.