Belongs to, and the mathematical symbol is "∈", indicating the relationship between elements and sets. If a∈A, then A belongs to set A, and A is an element in set A ... If A? A, then A does not belong to set A, and A is not an element in set A. In solid geometry, the symbol "∈" is used to represent the positional relationship between points, lines and surfaces.

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Set, for short, is a basic concept in mathematics and the main research object of set theory. The basic theory of set theory was founded in19th century. The simplest statement about set is the definition in naive set theory (the most primitive set theory), that is, set is "a certain pile of things" and the "things" in set are called elements.

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. The number of elements in a set is called the cardinality of the set.

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