1. Natural numbers are numbers used to measure the number of things or express the order of things, that is, the numbers 0, 1, 2, 3, 4, ... Natural numbers start from 0 and form an infinite group one by one.

2.Integer is a number indicating the number of objects, and 0 indicates that there are 0 objects. Integer is the most basic mathematical tool that human beings can master, and all integers form an integer set.

3. Positive integer, an integer greater than 0.

4. Rational numbers, integers and fractions are collectively called rational numbers, and the set of rational numbers can be represented by the capital black orthographic symbol Q, which definitely does not refer to rational numbers.

5. Real numbers, rational numbers and irrational numbers are collectively called positive real numbers, 0 and negative real numbers.

Extended data:

Representatives of other collections:

Z: integer set {…,-1, 0, 1, …}.

Q: Rational number set.

R+: set of positive real numbers.

R-: negative real number set.

C: complex set.

An empty set (a collection without any elements).

Q+: A set of positive rational numbers.

Q-: set of negative rational numbers.

The subtraction of natural numbers is not closed. It is closed unless the minuend is greater than the minuend. For example, 1 1 cannot subtract 26. In this case, use one of two methods:

(1) says that 26 cannot be subtracted from 1 1;

(2) If the answer is an integer representing a negative number, then the result of subtracting 26 from 1 1 is-15.

The subtraction of real numbers is defined as the addition of signed numbers. Specifically, one number subtracts the negative number of another number. And then we have three? π= 3 +(? π)。 This helps to avoid introducing "new" operators such as subtraction, thus maintaining the "simplicity" of real numbers.