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In topology and mathematics in general, the **boundary** of a subset *S* of a topological space *X* is the set of points which can be approached both from *S* and from the outside of *S*. More precisely, it is the set of points in the closure of not belonging to the interior of An element of the boundary of is called a **boundary point** of The term **boundary operation** refers to finding or taking the boundary of a set. Notations used for boundary of a set include and Some authors (for example Willard, in *General Topology*) use the term **frontier** instead of boundary in an attempt to avoid confusion with a different definition used in algebraic topology and the theory of manifolds. Despite widespread acceptance of the meaning of the terms boundary and frontier, they have sometimes been used to refer to other sets. For example, *Metric Spaces* by E. T. Copson uses the term boundary to refer to Hausdorff's **border**, which is defined as the intersection of a set with its boundary.^{ [1] } Hausdorff also introduced the term **residue**, which is defined as the intersection of a set with the closure of the border of its complement.^{ [2] }

- Common definitions
- Properties
- Examples
- Characterizations and general examples
- Concrete examples
- Boundary of an open ball vs. its surrounding sphere
- Boundary of a boundary
- See also
- Notes
- Citations
- References

A connected component of the boundary of is called a **boundary component** of

There are several equivalent definitions for the *boundary* of a subset of a topological space which will be denoted by or simply if is understood:

- It is the closure of minus the interior of in :
where denotes the closure of in and denotes the topological interior of in

- It is the intersection of the closure of with the closure of its complement:
- It is the set of points such that every neighborhood of contains at least one point of and at least one point not of :

A *boundary point* of a set refers to any element of that set's boundary. The boundary defined above is sometimes called the set's *topological boundary* to distinguish it from other similarly named notions such as the boundary of a manifold with boundary or the boundary of a manifold with corners, to name just a few example.

The closure of a set equals the union of the set with its boundary:

where denotes the closure of in A set is closed if and only if it contains its boundary, and open if and only if it is disjoint from its boundary. The boundary of a set is closed;^{ [3] } this follows from the formula which expresses as the intersection of two closed subsets of

("Trichotomy") Given any subset each point of lies in exactly one of the three sets and Said differently,

and these three sets are pairwise disjoint. Consequently, if these set are not empty^{ [note 1] } then they form a partition of

A point is a boundary point of a set if and only if every neighborhood of contains at least one point in the set and at least one point not in the set. The boundary of the interior of a set as well as the boundary of the closure of a set are both contained in the boundary of the set.

*Conceptual Venn diagram showing the relationships among different points of a subset of = set of limit points of set of boundary points of area shaded green = set of interior points of area shaded yellow = set of isolated points of areas shaded black = empty sets. Every point of is either an interior point or a boundary point. Also, every point of is either an accumulation point or an isolated point. Likewise, every boundary point of is either an accumulation point or an isolated point. Isolated points are always boundary points.*

The boundary of a set is equal to the boundary of the set's complement:

If is a dense open subset of then

The interior of the boundary of a closed set is the empty set.^{ [proof 1] } Consequently, the interior of the boundary of the closure of a set is the empty set. The interior of the boundary of an open set is also the empty set.^{ [proof 2] } Consequently, the interior of the boundary of the interior of a set is the empty set. In particular, if is a closed or open subset of then there does not exist any non-empty subset such that is also an open subset of This fact is important for the definition and use of nowhere dense subsets, meager subsets, and Baire spaces.

A set is the boundary of some open set if and only if it is closed and nowhere dense. The boundary of a set is empty if and only if the set is both closed and open (that is, a clopen set).

Consider the real line with the usual topology (that is, the topology whose basis sets are open intervals) and the subset of rational numbers (whose topological interior in is empty). Then

These last two examples illustrate the fact that the boundary of a dense set with empty interior is its closure. They also show that it is possible for the boundary of a subset to contain a non-empty open subset of ; that is, for the interior of in to be non-empty. However, a *closed* subset's boundary always has an empty interior.

In the space of rational numbers with the usual topology (the subspace topology of ), the boundary of where is irrational, is empty.

The boundary of a set is a topological notion and may change if one changes the topology. For example, given the usual topology on the boundary of a closed disk is the disk's surrounding circle: If the disk is viewed as a set in with its own usual topology, that is, then the boundary of the disk is the disk itself: If the disk is viewed as its own topological space (with the subspace topology of ), then the boundary of the disk is empty.

This example demonstrates that the topological boundary of an open ball of radius is *not* necessarily equal to the corresponding sphere of radius (centered at the same point); it also shows that the closure of an open ball of radius is *not* necessarily equal to the closed ball of radius (again centered at the same point). Denote the usual Euclidean metric on by

which induces on the usual Euclidean topology. Let denote the union of the -axis with the unit circle centered at the origin ; that is, which is a topological subspace of whose topology is equal to that induced by the (restriction of) the metric In particular, the sets and are all closed subsets of and thus also closed subsets of its subspace Henceforth, unless it clearly indicated otherwise, every open ball, closed ball, and sphere should be assumed to be centered at the origin and moreover, only the metric space will be considered (and not its superspace ); this being a path-connected and locally path-connected complete metric space.

Denote the open ball of radius in by so that when then

is the open sub-interval of the -axis strictly between and The unit sphere in ("unit" meaning that its radius is ) is

while the closed unit ball in is the union of the open unit ball and the unit sphere centered at this same point:

However, the topological boundary and topological closure in of the open unit ball are:

In particular, the open unit ball's topological boundary is a *proper* subset of the unit sphere in And the open unit ball's topological closure is a proper subset of the closed unit ball in The point for instance, cannot belong to because there does not exist a sequence in that converges to it; the same reasoning generalizes to also explain why no point in outside of the closed sub-interval belongs to Because the topological boundary of the set is always a subset of 's closure, it follows that must also be a subset of

In any metric space the topological boundary in of an open ball of radius centered at a point is always a subset of the sphere of radius centered at that same point ; that is,

always holds.

Moreover, the unit sphere in contains which is an open subset of ^{ [proof 3] } This shows, in particular, that the unit sphere in contains an *non-empty open* subset of

For any set where denotes the superset with equality holding if and only if the boundary of has no interior points, which will be the case for example if is either closed or open. Since the boundary of a set is closed, for any set The boundary operator thus satisfies a weakened kind of idempotence.

In discussing boundaries of manifolds or simplexes and their simplicial complexes, one often meets the assertion that the boundary of the boundary is always empty. Indeed, the construction of the singular homology rests critically on this fact. The explanation for the apparent incongruity is that the topological boundary (the subject of this article) is a slightly different concept from the boundary of a manifold or of a simplicial complex. For example, the boundary of an open disk viewed as a manifold is empty, as is its topological boundary viewed as a subset of itself, while its topological boundary viewed as a subset of the real plane is the circle surrounding the disk. Conversely, the boundary of a closed disk viewed as a manifold is the bounding circle, as is its topological boundary viewed as a subset of the real plane, while its topological boundary viewed as a subset of itself is empty. In particular, the topological boundary depends on the ambient space, while the boundary of a manifold is invariant.

- See the discussion of boundary in topological manifold for more details.
- Boundary of a manifold
- Bounding point – Mathematical concept related to subsets of vector spaces
- Closure (topology)
- Exterior (topology) – The largest open subset that is "outside of" a given subset.
- Interior (topology)
- Nowhere dense set
- Lebesgue's density theorem, for measure-theoretic characterization and properties of boundary
- Surface (topology) – Two-dimensional manifold

- ↑ Let be a closed subset of so that and thus also If is an open subset of such that then (because ) so that (because by definition, is the largest open subset of contained in ). But implies that Thus is simultaneously a subset of and disjoint from which is only possible if
- ↑ Let be an open subset of so that Let so that which implies that If then pick so that Because is an open neighborhood of in and the definition of the topological closure implies that which is a contradiction. Alternatively, if is open in then is closed in so that by using the general formula and the fact that the interior of the boundary of a closed set (such as ) is empty, it follows that
- ↑ The -axis is closed in because it is a product of two closed subsets of Consequently, is an open subset of Because has the subspace topology induced by the intersection is an open subset of

- ↑ Hausdorff, Felix (1914).
*Grundzüge der Mengenlehre*. Leipzig: Veit. p. 214. ISBN 978-0-8284-0061-9. Reprinted by Chelsea in 1949. - ↑ Hausdorff, Felix (1914).
*Grundzüge der Mengenlehre*. Leipzig: Veit. p. 281. ISBN 978-0-8284-0061-9. Reprinted by Chelsea in 1949. - ↑ Mendelson, Bert (1990) [1975].
*Introduction to Topology*(Third ed.). Dover. p. 86. ISBN 0-486-66352-3.Corollary 4.15 For each subset is closed.

In mathematics, a **topological space** is, roughly speaking, a geometrical space in which *closeness* is defined but, generally, cannot be measured by a numeric distance. More specifically, a topological space is a set of points, along with a set of neighbourhoods for each point, satisfying a set of axioms relating points and neighbourhoods.

In mathematics, **open sets** are a generalization of open intervals in the real line. In a metric space—that is, when a distance is defined—open sets are the sets that, with every point P, contain all points that are sufficiently near to P.

In mathematics, the **closure** of a subset *S* of points in a topological space consists of all points in *S* together with all limit points of *S*. The closure of *S* may equivalently be defined as the union of *S* and its boundary, and also as the intersection of all closed sets containing *S*. Intuitively, the closure can be thought of as all the points that are either in *S* or "near" *S*. A point which is in the closure of *S* is a point of closure of *S*. The notion of closure is in many ways dual to the notion of interior.

In mathematics, a **topological vector space** is one of the basic structures investigated in functional analysis. A topological vector space is a vector space which is also a topological space, this implies that vector space operations be continuous functions. More specifically, its topological space has a uniform topological structure, allowing a notion of uniform convergence.

In geometry, topology, and related branches of mathematics, a **closed set** is a set whose complement is an open set. In a topological space, 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 under the limit operation. This should not be confused with a closed manifold.

In mathematics, a subset of a topological space is called **nowhere dense** or **rare** if its closure has empty interior. In a very loose sense, it is a set whose elements are not tightly clustered anywhere. For example, the integers are nowhere dense among the reals, whereas an open ball is not.

In mathematics, specifically in topology, the **interior** of a subset S of a topological space X is the union of all subsets of S that are open in X. A point that is in the interior of S is an **interior point** of S.

In topology and related branches of mathematics, the **Kuratowski closure axioms** are a set of axioms that can be used to define a topological structure on a set. They are equivalent to the more commonly used open set definition. They were first formalized by Kazimierz Kuratowski, and the idea was further studied by mathematicians such as Wacław Sierpiński and António Monteiro, among others.

In mathematics, more specifically in topology, an **open map** is a function between two topological spaces that maps open sets to open sets. That is, a function is open if for any open set in the image is open in Likewise, a **closed map** is a function that maps closed sets to closed sets. A map may be open, closed, both, or neither; in particular, an open map need not be closed and vice versa.

In mathematics, more specifically in point-set topology, the **derived set** of a subset of a topological space is the set of all limit points of It is usually denoted by

In linear algebra and related areas of mathematics a **balanced set**, **circled set** or **disk** in a vector space is a set such that for all scalars satisfying

A **semiregular space** is a topological space whose regular open sets form a base.

In the mathematical theory of functional analysis, the **Krein–Milman theorem** is a proposition about compact convex sets in locally convex topological vector spaces (TVSs).

In topology and related fields of mathematics, a **sequential space** is a topological space that satisfies a very weak axiom of countability.

In the field of topology, a **Fréchet–Urysohn space** is a topological space with the property that for every subset the closure of in is identical to the *sequential* closure of in Fréchet–Urysohn spaces are a special type of sequential space.

In functional analysis, a branch of mathematics, the **algebraic interior** or **radial kernel** of a subset of a vector space is a refinement of the concept of the interior. It is the subset of points contained in a given set with respect to which it is absorbing, i.e. the radial points of the set. The elements of the algebraic interior are often referred to as **internal points**.

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.

In functional analysis, two methods of constructing normed spaces from disks were systematically employed by Alexander Grothendieck to define nuclear operators and nuclear spaces. One method is used if the disk is bounded: in this case, the **auxiliary normed space** is with norm The other method is used if the disk is absorbing: in this case, the auxiliary normed space is the quotient space If the disk is both bounded and absorbing then the two auxiliary normed spaces are canonically isomorphic.

In the mathematical field of set theory, an **ultrafilter** is a *maximal proper filter*: it is a filter on a given non-empty set which is a certain type of non-empty family of subsets of that is not equal to the power set of and that is also "maximal" in that there does not exist any other proper filter on that contains it as a proper subset. Said differently, a proper filter is called an ultrafilter if there exists *exactly one* proper filter that contains it as a subset, that proper filter (necessarily) being itself.

- Munkres, J. R. (2000).
*Topology*. Prentice-Hall. ISBN 0-13-181629-2. - Willard, S. (1970).
*General Topology*. Addison-Wesley. ISBN 0-201-08707-3. - van den Dries, L. (1998).
*Tame Topology*. ISBN 978-0521598385.

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