In

_{σ} sets. These sets need not be closed.

$[1,\; +\backslash infty)$ is closed.
* The Cantor set is an unusual closed set in the sense that it consists entirely of boundary points and is nowhere dense.
* Singleton points (and thus finite sets) are closed in T1 space, Tgeometry
Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is c ...

, topology
In mathematics, topology (from the Greek words , and ) is concerned with the properties of a geometric object that are preserved under continuous deformations, such as stretching, twisting, crumpling, and bending; that is, without closing ...

, and related branches of mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...

, a closed set is a set
Set, The Set, SET or SETS may refer to:
Science, technology, and mathematics Mathematics
*Set (mathematics), a collection of elements
*Category of sets, the category whose objects and morphisms are sets and total functions, respectively
Electro ...

whose complement is an open set. In a topological space
In mathematics, a topological space is, roughly speaking, a geometrical space in which closeness is defined but cannot necessarily be measured by a numeric distance. More specifically, a topological space is a set whose elements are called poin ...

, a closed set can be defined as a set which contains all its limit points. In a complete metric space, a closed set is a set which is closed
Closed may refer to:
Mathematics
* Closure (mathematics), a set, along with operations, for which applying those operations on members always results in a member of the set
* Closed set, a set which contains all its limit points
* Closed interval, ...

under the limit
Limit or Limits may refer to:
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* Limit (music), a way to characterize harmony
* "Limit" (song), a 2016 single by Luna Sea
* "Limits", a 2019 ...

operation.
This should not be confused with a closed manifold.
Equivalent definitions

By definition, a subset $A$ of atopological space
In mathematics, a topological space is, roughly speaking, a geometrical space in which closeness is defined but cannot necessarily be measured by a numeric distance. More specifically, a topological space is a set whose elements are called poin ...

$(X,\; \backslash tau)$ is called if its complement $X\; \backslash setminus\; A$ is an open subset of $(X,\; \backslash tau)$; that is, if $X\; \backslash setminus\; A\; \backslash in\; \backslash tau.$ A set is closed in $X$ if and only if it is equal to its closure in $X.$ Equivalently, a set is closed if and only if it contains all of its limit points. Yet another equivalent definition is that a set is closed if and only if it contains all of its boundary points.
Every subset $A\; \backslash subseteq\; X$ is always contained in its (topological) closure in $X,$ which is denoted by $\backslash operatorname\_X\; A;$ that is, if $A\; \backslash subseteq\; X$ then $A\; \backslash subseteq\; \backslash operatorname\_X\; A.$ Moreover, $A$ is a closed subset of $X$ if and only if $A\; =\; \backslash operatorname\_X\; A.$
An alternative characterization of closed sets is available via sequence
In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called ''elements'', or ''terms''). The number of elements (possibly infinite) is called ...

s and nets. A subset $A$ of a topological space $X$ is closed in $X$ if and only if every limit
Limit or Limits may refer to:
Arts and media
* ''Limit'' (manga), a manga by Keiko Suenobu
* ''Limit'' (film), a South Korean film
* Limit (music), a way to characterize harmony
* "Limit" (song), a 2016 single by Luna Sea
* "Limits", a 2019 ...

of every net of elements of $A$ also belongs to $A.$ In a first-countable space (such as a metric space), it is enough to consider only convergent sequence
In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called ''elements'', or ''terms''). The number of elements (possibly infinite) is called ...

s, instead of all nets. One value of this characterization is that it may be used as a definition in the context of convergence space
In mathematics, a convergence space, also called a generalized convergence, is a set together with a relation called a that satisfies certain properties relating elements of ''X'' with the family of filters on ''X''. Convergence spaces generaliz ...

s, which are more general than topological spaces. Notice that this characterization also depends on the surrounding space $X,$ because whether or not a sequence or net converges in $X$ depends on what points are present in $X.$
A point $x$ in $X$ is said to be a subset $A\; \backslash subseteq\; X$ if $x\; \backslash in\; \backslash operatorname\_X\; A$ (or equivalently, if $x$ belongs to the closure of $A$ in the topological subspace $A\; \backslash cup\; \backslash ,$ meaning $x\; \backslash in\; \backslash operatorname\_\; A$ where $A\; \backslash cup\; \backslash $ is endowed with the subspace topology induced on it by $X$In particular, whether or not $x$ is close to $A$ depends only on the subspace $A\; \backslash cup\; \backslash $ and not on the whole surrounding space (e.g. $X,$ or any other space containing $A\; \backslash cup\; \backslash $ as a topological subspace).).
Because the closure of $A$ in $X$ is thus the set of all points in $X$ that are close to $A,$ this terminology allows for a plain English description of closed subsets:
:a subset is closed if and only if it contains every point that is close to it.
In terms of net convergence, a point $x\; \backslash in\; X$ is close to a subset $A$ if and only if there exists some net (valued) in $A$ that converges to $x.$
If $X$ is a topological subspace of some other topological space $Y,$ in which case $Y$ is called a of $X,$ then there exist some point in $Y\; \backslash setminus\; X$ that is close to $A$ (although not an element of $X$), which is how it is possible for a subset $A\; \backslash subseteq\; X$ to be closed in $X$ but to be closed in the "larger" surrounding super-space $Y.$
If $A\; \backslash subseteq\; X$ and if $Y$ is topological super-space of $X$ then $A$ is always a (potentially proper) subset of $\backslash operatorname\_Y\; A,$ which denotes the closure of $A$ in $Y;$ indeed, even if $A$ is a closed subset of $X$ (which happens if and only if $A\; =\; \backslash operatorname\_X\; A$), it is nevertheless still possible for $A$ to be a proper subset of $\backslash operatorname\_Y\; A.$ However, $A$ is a closed subset of $X$ if and only if $A\; =\; X\; \backslash cap\; \backslash operatorname\_Y\; A$ for some (or equivalently, for every) topological super-space $Y$ of $X.$
Closed sets can also be used to characterize continuous functions: a map $f\; :\; X\; \backslash to\; Y$ is continuous
Continuity or continuous may refer to:
Mathematics
* Continuity (mathematics), the opposing concept to discreteness; common examples include
** Continuous probability distribution or random variable in probability and statistics
** Continuous g ...

if and only if $f\backslash left(\; \backslash operatorname\_X\; A\; \backslash right)\; \backslash subseteq\; \backslash operatorname\_Y\; (f(A))$ for every subset $A\; \backslash subseteq\; X$; this can be reworded in plain English as: $f$ is continuous if and only if for every subset $A\; \backslash subseteq\; X,$ $f$ maps points that are close to $A$ to points that are close to $f(A).$ Similarly, $f$ is continuous at a fixed given point $x\; \backslash in\; X$ if and only if whenever $x$ is close to a subset $A\; \backslash subseteq\; X,$ then $f(x)$ is close to $f(A).$
More about closed sets

The notion of closed set is defined above in terms of open sets, a concept that makes sense fortopological space
In mathematics, a topological space is, roughly speaking, a geometrical space in which closeness is defined but cannot necessarily be measured by a numeric distance. More specifically, a topological space is a set whose elements are called poin ...

s, as well as for other spaces that carry topological structures, such as metric space
In mathematics, a metric space is a set together with a notion of '' distance'' between its elements, usually called points. The distance is measured by a function called a metric or distance function. Metric spaces are the most general setti ...

s, differentiable manifolds, uniform spaces, and gauge spaces.
Whether a set is closed depends on the space in which it is embedded. However, the compact
Compact as used in politics may refer broadly to a pact or treaty; in more specific cases it may refer to:
* Interstate compact
* Blood compact, an ancient ritual of the Philippines
* Compact government, a type of colonial rule utilized in Britis ...

Hausdorff space
In topology and related branches of mathematics, a Hausdorff space ( , ), separated space or T2 space is a topological space where, for any two distinct points, there exist neighbourhoods of each which are disjoint from each other. Of the ma ...

s are " absolutely closed", in the sense that, if you embed a compact Hausdorff space $D$ in an arbitrary Hausdorff space $X,$ then $D$ will always be a closed subset of $X$; the "surrounding space" does not matter here. Stone–Čech compactification, a process that turns a completely regular Hausdorff space into a compact Hausdorff space, may be described as adjoining limits of certain nonconvergent nets to the space.
Furthermore, every closed subset of a compact space is compact, and every compact subspace of a Hausdorff space is closed.
Closed sets also give a useful characterization of compactness: a topological space $X$ is compact if and only if every collection of nonempty closed subsets of $X$ with empty intersection admits a finite subcollection with empty intersection.
A topological space $X$ is disconnected if there exist disjoint, nonempty, open subsets $A$ and $B$ of $X$ whose union is $X.$ Furthermore, $X$ is totally disconnected if it has an open basis consisting of closed sets.
Properties

A closed set contains its own boundary. In other words, if you are "outside" a closed set, you may move a small amount in any direction and still stay outside the set. Note that this is also true if the boundary is the empty set, e.g. in the metric space of rational numbers, for the set of numbers of which the square is less than $2.$ * Any intersection of any family of closed sets is closed (this includes intersections of infinitely many closed sets) * The union of closed sets is closed. * The empty set is closed. * The whole set is closed. In fact, if given a set $X$ and a collection $\backslash mathbb\; \backslash neq\; \backslash varnothing$ of subsets of $X$ such that the elements of $\backslash mathbb$ have the properties listed above, then there exists a unique topology $\backslash tau$ on $X$ such that the closed subsets of $(X,\; \backslash tau)$ are exactly those sets that belong to $\backslash mathbb.$ The intersection property also allows one to define the closure of a set $A$ in a space $X,$ which is defined as the smallest closed subset of $X$ that is a superset of $A.$ Specifically, the closure of $X$ can be constructed as the intersection of all of these closed supersets. Sets that can be constructed as the union of countably many closed sets are denoted FExamples

* The closed interval $;\; href="/html/ALL/l/,\_b.html"\; ;"title=",\; b">,\; b$real number
In mathematics, a real number is a number that can be used to measure a ''continuous'' one-dimensional quantity such as a distance, duration or temperature. Here, ''continuous'' means that values can have arbitrarily small variations. Every ...

s is closed. (See for an explanation of the bracket and parenthesis set notation.)
* The unit interval
In mathematics, the unit interval is the closed interval , that is, the set of all real numbers that are greater than or equal to 0 and less than or equal to 1. It is often denoted ' (capital letter ). In addition to its role in real analysis ...

$;\; href="/html/ALL/l/,\_1.html"\; ;"title=",\; 1">,\; 1$rational number
In mathematics, a rational number is a number that can be expressed as the quotient or fraction of two integers, a numerator and a non-zero denominator . For example, is a rational number, as is every integer (e.g. ). The set of all ra ...

s between $0$ and $1$ (inclusive) is closed in the space of rational numbers, but $;\; href="/html/ALL/l/,\_1.html"\; ;"title=",\; 1">,\; 1$ is not closed in the real numbers.
* Some sets are neither open nor closed, for instance the half-open interval $;\; href="/html/ALL/l/,\_1)$_in_the_real_numbers.
*_Some_sets_are_both_open_and_closed_and_are_called_clopen_sets.
*_The_Line_(geometry)#Ray.html" "title="clopen_sets.html" ;"title=", 1) in the real numbers.
* Some sets are both open and closed and are called clopen sets">, 1) in the real numbers.
* Some sets are both open and closed and are called clopen sets.
* The Line (geometry)#Ray">ray
Ray may refer to:
Fish
* Ray (fish), any cartilaginous fish of the superorder Batoidea
* Ray (fish fin anatomy), a bony or horny spine on a fin
Science and mathematics
* Ray (geometry), half of a line proceeding from an initial point
* Ray (gr ...See also

* * * * * * *Notes

References

* * * * {{DEFAULTSORT:Closed Set General topology