A set is the sum of some different objects in a certain range, and each object in this set is called an element. We call a set that contains no elements an empty set.

Attribution means that an element appears in a set, then this element belongs to this set.

Inclusion and being included are a set of similar concepts, similar to ≤ and ≥ To understand this concept, we must first understand what is a subset. If any element in set 1 is an element in set 2, then set 1 is a subset of set 2, then set 1 is included in set 2, and set 2 contains set 1.

True inclusion and true inclusion are similar in meaning to true inclusion and inclusion. The difference is that every element in true inclusion and true inclusion set 1 can be found in set 2, but set 1 is not exactly the same as set 2, and set 1 is the proper subset of set 2. Then the set 1 is really contained in set 2, and set 2 really contains set 65438+.