Generally speaking, if a set contains all the elements involved in the problem we are studying, it is called a complete set, usually denoted as U. In mathematics, especially in the application of set theory and mathematical foundation, the whole class (if it is a set, it is a complete set) is about such a class, which (to some extent) contains all the research objects and sets.

Extended data

1, n: non-negative integer set or natural number set {0, 1, 2,3, ...}

2, N* or N+: positive integer set {1, 2, 3, ...}

3, z: integer set {…,-1, 0, 1, …}

4. Q: Rational number set

5.Q+: Positive Rational Number Set

6.Q-: set of negative rational numbers

7.r: set of real numbers (including rational numbers and irrational numbers)

8.R+: positive real number set

9.R-: negative real number set

10, c: complex set

1 1、? : empty set (a set without any elements)

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