∈ is a symbol, which means that X can be found in the set D.

In modern mathematics set theory, elements are every object that constitutes a set. A collection consists of elements, and each object that makes up the collection is also called an element.

For example, 1, 2,3 in the set {1, 2,3} are all elements of the set.

characteristic

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. Interrelation

Any two elements in a collection are considered 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. Chaos

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.