Symbolic language of subsets: if a∈A has a∈B, then AB.

Proper subset's symbolic language: If the set AB has the element x∈B and the element X does not belong to the set A, then the set A is the proper subset of the set B, and it is recorded as AB.

When there are two sets, namely set A and set B, if all the elements contained in set A can be found from set B, we can say that set A is a subset of set B. ..

For example, if the complete set I is {1, 2,3}, then its subsets can be {1}, {2}, {3}, {1, 2}, {1, 3} and {2,3}. And its proper subset can only be {1}, {2}, {3}, {1, 2}, {1, 3}, {2,3},? . Its non-empty proper subset can only be {1}, {2}, {3}, {1, 2}, {1, 3}, {2,3}.