In set theory, the intersection of two sets is the set of elements that they have in common:
The intersection operation is:
The intersection of any finite number of sets may be defined inductively, as
The intersection of a general family of sets Xλ as λ ranges over a general index set Λ may be written in similar notation as
We may drop the indexing notation and define the intersection of a set to be the set of elements contained in all the elements of that set:
In this notation the intersection of two sets A and B may be expressed as
The correct definition of the intersection of the empty set needs careful consideration.