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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]}

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;

("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

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.

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

A set is a dense open subset of if and only if

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

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 a *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.