For example, the mapping from A to B depends on whether each element in set A has an image in set B (different elements in a can have the same image in set B). If yes, this correspondence is the mapping from A to B. Conversely, if an element in A has no image under this correspondence, it is not the mapping from A to B. You don't need to complete the elements in set B, such as: A = {1, 2,3}, b={a, b, c, d}. If every element in A is If 1 and 2 in A both correspond to B, and 3 does not correspond to any element in B, it is not a mapping from A to B. ..