In mathematics, the intersection of two sets A and B is the set that contains all elements of A that also belong to B (or equivalently, all elements of B that also belong to A), but no other elements.
The intersection of A and B is written "A ∩ B". Formally:
More generally, one can take the intersection of several sets at once. The intersection of A, B, C, and D, for example, is A ∩ B ∩ C ∩ D = A ∩ (B ∩ (C ∩ D)). Intersection is an associative operation; thus, A ∩ (B ∩ C) = (A ∩ B) ∩ C.
The most general notion is the intersection of an arbitrary nonempty collection of sets. If M is a nonempty set whose elements are themselves sets, then x is an element of the intersection of M if and only if for every element A of M, x is an element of A. In symbols:
The notation for this last concept can vary considerably. Hardcore set theorists will simply write "∩M", while most people will instead write "∩_{A∈M }A". The latter notation can be generalised to "∩_{i∈I} A_{i}", which refers to the intersection of the collection {A_{i} : i ∈ I}. Here I is a nonempty set, and A_{i} is a set for every i in I.
In the case that the index set I is the set of natural numbers, you might see notation analogous to that of an infinite series:
Finally, let us note that whenever the symbol "∩" is placed before other symbols instead of between them, it should be of a larger size.
(Eventually this will be available in HTML as the character entity ⋂
, but until then, try <big>∩</big>
.)
See also: Basic set theory