ρ2={(a, b)| There is an element c∈A, so (a, c) ∈ ▌▌▌1,and (c, a) ∈▌1}

When it is finite, the set A is called finite, otherwise it is infinite. Generally speaking, a set with finite elements is called a finite set, and a set with infinite elements is called an infinite set.

Extended data:

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.

The method of enumerating the elements of a collection one by one. For example, the three primary colors in optics can be represented by the set {red, green and blue}; The set A composed of four letters A, B, C and D can be represented by a = {a, B, C, d}, and so on.