For example, two sets A {1, 2,3} and B {1, 2,4,5}.

Then A∩B represents the elements in the set AB***, that is, {1, 2}.

AUB refers to all elements of two sets, and some elements are only calculated once, namely {1, 2, 3, 4, 5}.

Historical position

Set has unparalleled special importance in the field of mathematics. The foundation of set theory was laid by German mathematician Cantor in the 1970s of 19. After the efforts of a large number of outstanding scientists for half a century, it established its basic position in the modern mathematical theory system in the 1920s. It can be said that the achievements of all branches of modern mathematics are almost based on strict set theory.

The essence of alliance:

Empty set is the unit element of union operation, that is? ∪A=A = A. For any set A, an empty set can be regarded as the union of zero sets. Union and intersection satisfy the distribution law, and these three operations satisfy De Morgan's law. If the union operation is replaced by symmetric difference operation, the corresponding Boolean ring can be obtained.