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In mathematics, the **intersection** of two sets *A* and *B*, denoted by *A* ∩ *B*,^{ [1] }^{ [2] } is the set containing all elements of *A* that also belong to *B* (or equivalently, all elements of *B* that also belong to *A*).^{ [3] }

Intersection is written using the sign "∩" between the terms; that is, in infix notation. For example,

The intersection of more than two sets (generalized intersection) can be written as^{ [1] }

which is similar to capital-sigma notation.

For an explanation of the symbols used in this article, refer to the table of mathematical symbols.

The intersection of two sets *A* and *B*, denoted by *A* ∩ *B*,^{ [1] }^{ [4] } is the set of all objects that are members of both the sets *A* and *B*. In symbols,

That is, *x* is an element of the intersection *A* ∩ *B*, if and only if *x* is both an element of *A* and an element of *B*.^{ [4] }

For example:

- The intersection of the sets {1, 2, 3} and {2, 3, 4} is {2, 3}.
- The number 9 is
*not*in the intersection of the set of prime numbers {2, 3, 5, 7, 11, ...} and the set of odd numbers {1, 3, 5, 7, 9, 11, ...}, because 9 is not prime.

Intersection is an associative operation; that is, for any sets *A*, *B*, and *C*, one has *A* ∩ (*B* ∩ *C*) = (*A* ∩ *B*) ∩ *C*. Intersection is also commutative; for any *A* and *B*, one has *A* ∩ *B* = *B* ∩ *A.* It thus makes sense to talk about intersections of multiple sets. The intersection of *A*, *B*, *C*, and *D*, for example, is unambiguously written *A* ∩ *B* ∩ *C* ∩ *D*.

Inside a universe *U*, one may define the complement *A*^{c} of *A* to be the set of all elements of *U* not in *A*. Furthermore, the intersection of *A* and *B* may be written as the complement of the union of their complements, derived easily from De Morgan's laws:*A* ∩ *B* = (*A*^{c} ∪ *B*^{c})^{c}

We say that *A intersects (meets) B at an element x* if *x* belongs to *A* and *B*. We say that *A intersects (meets) B* if *A* intersects B at some element. *A* intersects *B* if their intersection is inhabited.

We say that *A and B are disjoint * if *A* does not intersect *B*. In plain language, they have no elements in common. *A* and *B* are disjoint if their intersection is empty, denoted .

For example, the sets {1, 2} and {3, 4} are disjoint, while the set of even numbers intersects the set of multiples of 3 at the multiples of 6.

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. Set theorists will sometimes write "⋂*M*", while others will instead write "⋂_{A∈M }*A*". The latter notation can be generalized 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, notation analogous to that of an infinite product may be seen:

When formatting is difficult, this can also be written "*A*_{1} ∩ *A*_{2} ∩ *A*_{3} ∩ ...". This last example, an intersection of countably many sets, is actually very common; for an example, see the article on σ-algebras.

Note that in the previous section, we excluded the case where *M* was the empty set (∅). The reason is as follows: The intersection of the collection *M* is defined as the set (see set-builder notation)

If *M* is empty, there are no sets *A* in *M*, so the question becomes "which *x*'s satisfy the stated condition?" The answer seems to be *every possible x*. When *M* is empty, the condition given above is an example of a vacuous truth. So the intersection of the empty family should be the universal set (the identity element for the operation of intersection),^{ [5] } but in standard (ZFC) set theory, the universal set does not exist.

In type theory however, *x* is of a prescribed type , so the intersection is understood to be of type (the type of sets whose elements are in ), and we can define to be the universal set of (the set whose elements are exactly all terms of type ).

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In algebra, a **prime ideal** is a subset of a ring that shares many important properties of a prime number in the ring of integers. The prime ideals for the integers are the sets that contain all the multiples of a given prime number, together with the zero ideal.

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In mathematics, specifically set theory, the **Cartesian product** of two sets *A* and *B*, denoted *A* × *B*, is the set of all ordered pairs (*a*, *b*) where *a* is in *A* and *b* is in *B*. In terms of set-builder notation, that is

In topology, a subfield of mathematics, *filters* are special families of subsets of a set that can be used to study topological spaces and define all basic topological notions such a convergence, continuity, compactness, and more. Filters also provide a common framework for defining various types of limits of functions such as limits from the left/right, to infinity, to a point or a set, and many others. Special types of filters called *ultrafilters* have many useful technical properties and they may often be used in place of arbitrary filters.

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*www.probabilitycourse.com*. Retrieved 2020-09-04. - ↑ Megginson, Robert E. (1998), "Chapter 1",
*An introduction to Banach space theory*, Graduate Texts in Mathematics,**183**, New York: Springer-Verlag, pp. xx+596, ISBN 0-387-98431-3

- Devlin, K. J. (1993).
*The Joy of Sets: Fundamentals of Contemporary Set Theory*(Second ed.). New York, NY: Springer-Verlag. ISBN 3-540-94094-4. - Munkres, James R. (2000). "Set Theory and Logic".
*Topology*(Second ed.). Upper Saddle River: Prentice Hall. ISBN 0-13-181629-2. - Rosen, Kenneth (2007). "Basic Structures: Sets, Functions, Sequences, and Sums".
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