It means set, and the things in it are called elements in the set!

{AI | 1 < I < n} This is also a set, which means that there are n elements in this set, namely a 1, a2, ..., An.

A set is the sum of things with certain properties. The "thing" here can be a person, an object or a mathematical element. For example: 1, scattered people or things gather together; Assemble: urgent.

Representation of set

1. Enumeration: commonly used to represent a finite set, in which all elements are listed one by one and written in braces. This method of representing a set is called enumeration. { 1,2,3,……}

2. Description: It is often used to represent an infinite set. The public * * * attribute of the elements in the collection is described by words, symbols or expressions and enclosed in braces. This method of representing a set is called description. {x|P}(x is the general form of the elements of this set, and p is the * * * same property of the elements of this set) For example, a set composed of positive real numbers less than π is expressed as {x | 0.