Mathematical n is a set of natural numbers in a set, which is a set of all non-negative integers, and there are infinite natural numbers. Natural numbers are signs of the same characteristics of all equivalent finite sets. Integers include natural numbers, so natural numbers must be integers and non-negative integers.

Features of this kit:

1. certainty: given a set, any element, whether it belongs to the set or not, must be one of them, and there is no ambiguity.

2. Mutuality: Any two elements in a set are considered to be different, that is, each element can only appear once. Sometimes it is necessary to describe the situation where the same element appears many times. You can use multiset, where elements are allowed to appear multiple times.

3. Disorder: In a set, the state of each element is the same, and the elements are out of order. You can define an order relation on the set. After defining the order relation, you can sort the elements according to the order relation. But as far as the characteristics of the set itself are concerned, there is no necessary order between elements.