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Natural numbers are numbers 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, ... natural numbers start from 0, one by one, forming an infinite set. 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 the 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.