These three natural numbers are multiplied by two, and natural numbers refer to numbers used to measure the number of things or express the order of things. That is, the number 0, 1, 2, 3, 4 represents the number. Natural numbers start from 0, one by one, forming an infinite group. Natural numbers are ordered and infinite. Divided into even and odd numbers, composite numbers and prime numbers.

But the results of subtraction and division are not always natural numbers, so subtraction and division operations are not always true in natural number sets. A number used to measure the number of things or to indicate the order of things. That is, the numbers represented by the numbers 0, 1, 2, 3, 4, ... the numbers representing the number of objects are called natural numbers, and the natural numbers form an infinite group one after another.

There are addition and multiplication operations in the set of natural numbers. The result of addition or multiplication of two natural numbers is still a natural number, and subtraction or division can also be done, but the result of subtraction and division is not necessarily a natural number, so subtraction and division operations are not always effective in the set of natural numbers.

Natural numbers are the most basic of all numbers that people know. In order to make the number system have a strict logical basis, mathematicians in the19th century established two equivalent theories of natural numbers: ordinal number theory and cardinal number theory, which made the concept, operation and related properties of natural numbers strictly discussed.