A, some specified objects together into a set, called a set, where each object is called an element. For example, the collection of Grade One and Grade Two, then all the students in Grade One and Grade Two form a collection, and each student is called the element of this collection.

2. Generally, sets are represented by uppercase letters, and elements are represented by lowercase letters.

Third, in a set, the state of each element is the same, and the elements are out of order.

4. The foundation of set theory was laid by German mathematician Cantor in 1970s of 19. After the efforts of a large number of scientists for half a century, it established its basic position in the modern mathematical theory system in the 1920s. It can be said that the achievements of all branches of modern mathematics are almost based on strict set theory.

5. The number of elements in the set is called the cardinality of the set, and the cardinality of the set A is called the card (a). When it is finite, the set A is called finite, otherwise it is infinite. Generally speaking, a set with finite elements is called a finite set, and a set with infinite elements is called an infinite set.