The members of these collection objects are called "elements" of the collection.

It is conditional that a group of objects can form a set, and the most important point is that the elements in the set must be certain. For example, "all tall people" cannot form a set, because tall people are not a definite concept; But "All Birthdays Today" is a collection. Even if you don't know who these people are, they are sure and unambiguous.

In naive set theory (high school students are the most basic concept and nature of naive set theory), set is an undefined concept. Therefore, it is enough to know the above concepts about sets.

If A and B are two sets, and each element in B is also an element of A, then B is called a "subset".

Note that the case of A = B is not excluded here. If B is a subset of A and B≠A, then B is called the "proper subset" of A..

For example, an integer constitutes a set and an odd number constitutes a set, then the integer set is a subset of the integer set itself, and the odd set is also a subset of the integer set, which is a proper subset.

If A and B are two sets, then the common elements of A and B (that is, elements belonging to both A and B) are selected as a set, which is called the intersection of A and B..

If A and B are two sets, then all the elements that A has or B has (or both A and B have) also form a set, which is called the "union set" of A and B.

Wait a minute.