Intersection

In set theory, the intersection of two sets is the set of elements that they have in common:

${\displaystyle A\cap B=\{x:x\in A\wedge x\in B\},}$

where ${\displaystyle \wedge }$ denotes logical and. Two sets are disjoint if their intersection is the empty set.

Properties[edit]

The intersection operation is:

• associative : ${\displaystyle (A\cap B)\cap C=A\cap (B\cap C)}$;
• commutative : ${\displaystyle A\cap B=B\cap A}$.

General intersections[edit]

Finite intersections[edit]

The intersection of any finite number of sets may be defined inductively, as

${\displaystyle {\displaystyle \bigcap _{i=1}^n}{X}_i=X_1\cap (X_2\cap (X_3\cap (\cdots X_n)\cdots ))).}$

Infinite intersections[edit]

The intersection of a general family of sets X_λ as λ ranges over a general index set Λ may be written in similar notation as

${\displaystyle \bigcap _{\lambda \in \mathrm{\Lambda }}{X}_{\lambda }=\{x:\mathrm{\forall }\lambda \in \mathrm{\Lambda },x\in X_{\lambda }\}.}$

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:

${\displaystyle \bigcap X=\{x:\mathrm{\forall }Y\in X,x\in Y\}.}$

In this notation the intersection of two sets A and B may be expressed as

${\displaystyle A\cap B=\bigcap \{A,B\}.}$

The correct definition of the intersection of the empty set needs careful consideration.

See also[edit]

• Union

References[edit]

• Paul Halmos (1960). Naive set theory. Van Nostrand Reinhold.  Section 4.
• Keith J. Devlin (1979). Fundamentals of Contemporary Set Theory. Springer-Verlag, 6,11. ISBN 0-387-90441-7.