Practical application: The collection of all China people, whose element is every China person. Generally, uppercase letters such as A, B, S, T, ... are used to represent a set, while lowercase letters such as A, B, X, Y, ... are used to represent elements in the set. If X is an element of the set S, it is said that X belongs to S and is marked as X ∈ S ... If Y is not an element of the set S, it is said that Y does not belong to S and is marked as Y? S

Set has unparalleled special importance in the field of mathematics.

The foundation of set theory was laid by German mathematician Cantor in the 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.

Extended data:

Features of this kit:

1, certainty

Given a set, any element, whether it belongs to the set or not, must be one of them, and there is no ambiguity.

2. Interrelation

Any two elements in a collection are considered different, that is, each element can only appear once. Sometimes it is necessary to describe the situation where the same element appears many times. You can use multiset, where elements are allowed to appear multiple times.

3. Chaos

In a set, the state of each element is the same and the elements are out of order. You can define an order relation on the set. After defining the order relation, you can sort the elements according to the order relation. But as far as the characteristics of the set itself are concerned, there is no necessary order between elements.

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