If all the elements in set A pass the corresponding rule f(x), then the only element that can be found in set B is equal to it, then these two sets A.

B plus the corresponding law constitutes a mapping. Remember, all three are indispensable. We call the elements in A the original phase, and the elements in B the phase. After the elements in A pass the corresponding laws, they can find the unique phase in B, but the elements in B don't have to find the original phase in A, even if they do, they are not necessarily unique.

A map that can form a function is called surjective. What is a surjection? That is, after the elements in A pass the corresponding laws, they can all find the unique phase in B set. Moreover, the elements in B can also find their corresponding original phases in A, but please remember that the corresponding original phases are not necessarily unique, but there can be many. . .