In the general set problem, the concepts of set and element are distinguished. The above two symbols are binary operators, and their usage is as follows:

A ∪ b: the set of all elements in a and b; For example, if a = {1, 2} and b = {2,3}, then a ∪ b = {1, 2,3};

A ∩ b: the set of the same elements in A and B; For example, a and b above, a ∩ b = {2};

In professional set theory, everything is a set, and the elements of the set are also sets. Therefore, these two symbols can be regarded as unary operators:

∪A:A: the union of all elements in a; For example, if A={a 1, a2, a3}, then ∪ a = a1∪ a2 ∪ a3;

∩ the intersection of all elements in a: a: a;