Union: indicates the method ∨.

In set theory, let A and B be two sets, which are composed of all elements belonging to set A and set B, called the intersection of set A and set B, and marked as A ∩ B.

Extended data operation

(1) If the intersection of two sets A and B is empty, they are said to have no common elements. . For example, the set {1, 2} and {3,4} do not intersect, and the writing {1, 2} ∩ {3,4} =? .

(2) The intersection of an arbitrary set and an empty set is an empty set, that is, A∩? =? .

(3) More generally, the intersection operation can be performed on multiple sets at the same time. For example, the intersection of sets a, b, c and d is A∩B∩C∩D=A∩[B∩(C ∩D)]. The intersection operation satisfies the associative law, that is, a ∩ (b ∩ c) = (a ∩ b) ∩ c c.

(4) The most abstract concept is the intersection of any nonempty set. If M is a non-empty set and its elements are also sets, then X belongs to the intersection of M if and only if it belongs to A for any element of M, this concept is the same as the above idea. For example, A∩B∩C is the intersection of sets {A, b, C} (it is sometimes clear when m is empty, please see the intersection of empty).