Whether U looks like a container or not, you can imagine that it puts things on both sides, and then the two parts are merged into one part, so "U" is called union.
N looks like a door, and this door is a special door. It only allows the intersecting parts, that is, the parts with the same points, and leaves the "heterogeneous" outside the door, so n is called intersection.
(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} (m is sometimes clear when it is empty, please see empty intersection).