1, the set of all positive integers is called positive integer set, and it is recorded as N*, Z+ or N+.
2. The set of all negative integers is called the set of negative integers, and is denoted as Z-.
3. A set composed of all nonnegative integers is called a nonnegative integer set (or natural number set), and is denoted as n..
4. The set composed of all integers is called integer set, which is denoted as Z. ..
5. The set of all real numbers is called the set of real numbers, which is denoted as R. ..
concept
A collection refers to a collection of concrete or abstract objects with certain attributes. Among them, these objects that make up the set are called elements of the set.
For example, the collection of all China people, its 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？