Existence means that any element in a random set exists as long as one of the sets is satisfied.

Set is a basic concept in mathematics and the research object of set theory. The basic theory of set theory was not founded until19th century. In the simplest way, defined by the most primitive set theory-naive set theory, a set is "a bunch of things". The "things" in a set are called elements.

A collection consists of one or more elements. If X is an element of set A, it is denoted as x ∈ a, and the elements in the set have three characteristics: 1. Determinism (the elements in a set must be definite) 2. Different from each other (the elements in the collection are different from each other. For example, if the set A={ 1, a}, then a cannot be equal to 1) 3. Out of order (the elements in the collection have no order. )