View source for Boundary (topology)

{{Short description|All points not part of the interior of a subset of a topological space}}
[[File:Runge theorem.svg|right|thumb|A set (in light blue) and its boundary (in dark blue).]]

In [[Topology|topology]] and [[Mathematics|mathematics]] in general, the '''boundary''' of a subset {{mvar|S}} of a [[Topological space|topological space]] {{mvar|X}} is the set of points in the [[Closure (topology)|closure]] of {{mvar|S}} not belonging to the [[Interior (topology)|interior]] of {{mvar|S}}. An element of the boundary of {{mvar|S}} is called a '''boundary point''' of {{mvar|S}}. The term '''boundary operation''' refers to finding or taking the boundary of a set.  Notations used for boundary of a set {{mvar|S}} include <math>\operatorname{bd}(S), \operatorname{fr}(S),</math>  and <math>\partial S</math>.

Some authors (for example Willard, in ''General Topology'') use the term '''frontier''' instead of boundary in an attempt to avoid confusion with a [[Manifold#Manifold with boundary|different definition]] used in [[Algebraic topology|algebraic topology]] and the theory of [[Manifold|manifold]]s.  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 [[Biography:Felix Hausdorff|Hausdorff]]'s '''border''', which is defined as the intersection of a set with its boundary.<ref>{{cite book|last=Hausdorff|first=Felix|year=1914|title=Grundzüge der Mengenlehre|publisher=Veit|place=Leipzig|page=[https://archive.org/details/grundzgedermen00hausuoft/page/214 214]|url=https://archive.org/details/grundzgedermen00hausuoft|isbn=978-0-8284-0061-9}} Reprinted by Chelsea in 1949.</ref> Hausdorff also introduced the term '''residue''', which is defined as the intersection of a set with the closure of the border of its complement.<ref>{{cite book|last=Hausdorff|first=Felix |year=1914|title=Grundzüge der Mengenlehre|publisher=Veit|place=Leipzig |page=[https://archive.org/details/grundzgedermen00hausuoft/page/281 281]|url=https://archive.org/details/grundzgedermen00hausuoft|isbn=978-0-8284-0061-9}} Reprinted by Chelsea in 1949.</ref>

== Definitions ==

There are several equivalent definitions for the '''boundary''' of a subset <math>S \subseteq X</math> of a topological space <math>X,</math> which will be denoted by <math>\partial_X S,</math> <math>\operatorname{Bd}_X S,</math> or simply <math>\partial S</math> if <math>X</math> is understood:
<ol start=1>
<li>It is the [[Closure (topology)|closure]] of <math>S</math> minus the [[Interior (topology)|interior]] of <math>S</math> in <math>X</math>:  <math display="block">\partial S ~:=~ \overline{S} \setminus \operatorname{int}_X S</math>
where <math>\overline{S} = \operatorname{cl}_X S</math> denotes the [[Closure (topology)|closure]] of <math>S</math> in <math>X</math> and <math>\operatorname{int}_X S</math> denotes the [[Interior (topology)|topological interior]] of <math>S</math> in <math>X.</math>
</li>
<li>It is the intersection of the closure of <math>S</math> with the closure of its [[Complement (set theory)|complement]]: <math display="block">\partial S ~:=~ \overline{S} \cap \overline{(X \setminus S)}</math></li>
<li>It is the set of points <math>p \in X</math> such that every neighborhood of <math>p</math> contains at least one point of <math>S</math> and at least one point not of <math>S</math>: <math display="block">\partial S ~:=~ \{ p \in X : \text{ for every neighborhood } O \text{ of } p, \ O \cap S \neq \varnothing \,\text{ and }\, O \cap (X \setminus S) \neq \varnothing \}.</math></li>
</ol>

A '''boundary point''' of a set is any element of that set's boundary. The boundary <math>\partial_X S</math> 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 examples.

A [[Connected space#Formal definition|connected component]] of the boundary of {{mvar|S}} is called a '''boundary component''' of {{mvar|S}}.

== Properties ==

The closure of a set <math>S</math> equals the union of the set with its boundary: 
<math display="block">\overline{S} = S \cup \partial_X S</math> 
where <math>\overline{S} = \operatorname{cl}_X S</math> denotes the [[Closure (topology)|closure]] of <math>S</math> in <math>X.</math> 
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 set|closed]];<ref>{{cite book|last=Mendelson|first=Bert|date=1990|orig-year=1975|title=Introduction to Topology|edition=Third|publisher=Dover|isbn=0-486-66352-3|page=86|quote=Corollary 4.15 For each subset <math>A,</math> <math>\operatorname{Bdry} (A)</math> is closed.}}</ref> this follows from the formula <math>\partial_X S ~:=~ \overline{S} \cap \overline{(X \setminus S)},</math> which expresses <math>\partial_X S</math> as the intersection of two closed subsets of <math>X.</math> 

("Trichotomy"){{Anchor|Trichotomy}}<!-- Linked to from [[Nowhere dense set]] --> Given any subset <math>S \subseteq X,</math> each point of <math>X</math> lies in exactly one of the three sets <math>\operatorname{int}_X S, \partial_X S,</math> and <math>\operatorname{int}_X (X \setminus S).</math>  Said differently, <math display="block">X ~=~ \left(\operatorname{int}_X S\right) \;\cup\; \left(\partial_X S\right) \;\cup\; \left(\operatorname{int}_X (X \setminus S)\right)</math> and these three sets are pairwise disjoint. Consequently, if these set are not empty<ref group=note>The condition that these sets be non-empty is needed because sets in a [[Partition of a set|partition]] are by definition required to be non-empty.</ref> then they form a [[Partition of a set|partition]] of <math>X.</math> 

A point <math>p \in X</math> is a boundary point of a set if and only if every neighborhood of <math>p</math> 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. 

<div class="center">
[[File:Accumulation And Boundary Points Of S.PNG]]<br/>
''Conceptual [[Venn diagram]] showing the relationships among different points of a subset <math>S</math> of <math>\R^n.</math> <math>A</math> = set of [[Limit point|limit point]]s of <math>S,</math> <math>B = </math> set of '''boundary points''' of <math>S,</math> area shaded green = set of interior points of <math>S,</math> area shaded yellow = set of [[Isolated point|isolated point]]s of <math>S,</math> areas shaded black = empty sets.  Every point of <math>S</math> is either an interior point or a boundary point.  Also, every point of <math>S</math> is either an accumulation point or an isolated point.  Likewise, every boundary point of <math>S</math> is either an accumulation point or an isolated point.  Isolated points are always boundary points.''
</div>

== Examples ==

=== Characterizations and general examples ===

A set and its complement have the same boundary:
<math display="block">\partial_X S = \partial_X (X \setminus S).</math>

A set <math>U</math> is a dense [[Open set|open]] subset of <math>X</math> if and only if <math>\partial_X U = X \setminus U.</math> 

The interior of the boundary of a closed set is empty.<ref group="proof">Let <math>S</math> be a closed subset of <math>X</math> so that <math>\overline{S} = S</math> and thus also <math>\partial_X S := \overline{S} \setminus \operatorname{int}_X S = S \setminus \operatorname{int}_X S.</math> If <math>U</math> is an open subset of <math>X</math> such that <math>U \subseteq \partial_X S</math> then  <math>U \subseteq S</math> (because <math>\partial_X S \subseteq S</math>) so that <math>U \subseteq \operatorname{int}_X S</math> (because [[Interior (topology)|by definition]], <math>\operatorname{int}_X S</math> is the largest open subset of <math>X</math> contained in <math>S</math>). But <math>U \subseteq \partial_X S = S \setminus \operatorname{int}_X S</math> implies that <math>U \cap \operatorname{int}_X S = \varnothing.</math> Thus <math>U</math> is simultaneously a subset of <math>\operatorname{int}_X S</math> and disjoint from <math>\operatorname{int}_X S,</math> which is only possible if <math>U = \varnothing.</math> [[Q.E.D.]]</ref> 
Consequently, the interior of the boundary of the closure of a set is empty. 
The interior of the boundary of an open set is also empty.<ref group="proof">Let <math>S</math> be an open subset of <math>X</math> so that <math>\partial_X S := \overline{S} \setminus \operatorname{int}_X S = \overline{S} \setminus S.</math> Let <math>U := \operatorname{int}_X \left(\partial_X S\right)</math> so that <math>U = \operatorname{int}_X \left(\partial_X S\right) \subseteq \partial_X S = \overline{S} \setminus S,</math> which implies that <math>U \cap S = \varnothing.</math> If <math>U \neq \varnothing</math> then pick <math>u \in U,</math> so that <math>u \in U \subseteq \partial_X S \subseteq \overline{S}.</math> Because <math>U</math> is an open neighborhood of <math>u</math> in <math>X</math> and <math>u \in \overline{S},</math> the definition of the [[Closure (topology)|topological closure]] <math>\overline{S}</math> implies that <math>U \cap S \neq \varnothing,</math> which is a contradiction. <math>\blacksquare</math> Alternatively, if <math>S</math> is open in <math>X</math> then <math>X \setminus S</math> is closed in <math>X,</math> so that by using the general formula <math>\partial_X S = \partial_X (X \setminus S)</math> and the fact that the interior of the boundary of a closed set (such as <math>X \setminus S</math>) is empty, it follows that <math>\operatorname{int}_X \partial_X S = \operatorname{int}_X \partial_X (X \setminus S) = \varnothing.</math> <math>\blacksquare</math></ref> 
Consequently, the interior of the boundary of the interior of a set is empty. 
In particular, if <math>S \subseteq X</math> is a closed or open subset of <math>X</math> then there does not exist any nonempty subset <math>U \subseteq \partial_X S</math> such that <math>U</math> is open in <math>X.</math> 
This fact is important for the definition and use of [[Nowhere dense set|nowhere dense subsets]], meager subsets, and [[Baire space]]s. 

A set is the boundary of some open set if and only if it is closed and [[Nowhere dense set|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|clopen set]]).

=== Concrete examples ===

[[File:Mandelbrot Components.svg|right|thumb|Boundary of hyperbolic components of [[Mandelbrot set|Mandelbrot set]]]]
Consider the real line <math>\R</math> with the usual topology (that is, the topology whose basis sets are open intervals) and <math>\Q,</math> the subset of rational numbers (whose [[Interior (topology)|topological interior]] in <math>\R</math> is empty).  Then

* <math>\partial (0,5) = \partial [0,5) = \partial (0,5] = \partial [0,5] = \{0, 5\}</math>
* <math>\partial \varnothing= \varnothing</math>
* <math>\partial \Q = \R</math>
* <math>\partial (\Q \cap [0, 1]) = [0, 1]</math>

These last two examples illustrate the fact that the boundary of a [[Dense set|dense set]] with empty interior is its closure. They also show that it is possible for the boundary <math>\partial S</math> of a subset <math>S</math> to contain a non-empty open subset of <math>X := \R</math>; that is, for the interior of <math>\partial S</math> in <math>X</math> to be non-empty. However, a {{em|closed}} subset's boundary always has an empty interior. 

In the space of rational numbers with the usual topology (the [[Subspace topology|subspace topology]] of <math>\R</math>), the boundary of <math>(-\infty, a),</math> where <math>a</math> is irrational, is empty.

The boundary of a set is a [[Topology|topological]] notion and may change if one changes the topology.  For example, given the usual topology on <math>\R^2,</math> the boundary of a closed disk <math>\Omega = \left\{(x, y) : x^2 + y^2 \leq 1 \right\}</math> is the disk's surrounding circle: <math>\partial \Omega = \left\{(x, y) : x^2 + y^2 = 1 \right\}.</math>  If the disk is viewed as a set in <math>\R^3</math> with its own usual topology, that is, <math>\Omega = \left\{(x, y, 0) : x^2 + y^2 \leq 1 \right\},</math> then the boundary of the disk is the disk itself: <math>\partial \Omega = \Omega.</math>  If the disk is viewed as its own topological space (with the subspace topology of <math>\R^2</math>), then the boundary of the disk is empty. 

=== Boundary of an open ball vs. its surrounding sphere ===

This example demonstrates that the topological boundary of an open ball of radius <math>r > 0</math> is {{em|not}} necessarily equal to the corresponding sphere of radius <math>r</math> (centered at the same point); it also shows that the closure of an open ball of radius <math>r > 0</math> is {{em|not}} necessarily equal to the closed ball of radius <math>r</math> (again centered at the same point). 
Denote the usual Euclidean metric on <math>\R^2</math> by
<math display="block">d((a, b), (x, y)) := \sqrt{(x - a)^2 + (y - b)^2}</math>
which induces on <math>\R^2</math> the usual [[Euclidean topology]]. 
Let <math>X \subseteq \R^2</math> denote the union of the <math>y</math>-axis <math>Y := \{ 0 \} \times \R</math> with the unit circle <math display="block">S^1 := \left\{ p \in \R^2 : d(p, \mathbf{0}) = 1 \right\} = \left\{ (x, y) \in \R^2 : x^2 + y^2 = 1 \right\}</math> centered at the origin <math>\mathbf{0} := (0, 0) \in \R^2</math>; that is, <math>X := Y \cup S^1,</math> which is a topological subspace of <math>\R^2</math> whose topology is equal to that induced by the (restriction of) the metric <math>d.</math> 
In particular, the sets <math>Y, S^1, Y \cap S^1 = \{ (0, \pm 1) \},</math> and <math>\{ 0 \} \times [-1, 1]</math> are all closed subsets of <math>\R^2</math> and thus also closed subsets of its subspace <math>X.</math> 
Henceforth, unless it clearly indicated otherwise, every open ball, closed ball, and sphere should be assumed to be centered at the origin <math>\mathbf{0} = (0, 0)</math> and moreover, only the [[Metric space|metric space]] <math>(X, d)</math> will be considered (and not its superspace <math>(\R^2, d)</math>); this being a path-connected and locally path-connected [[Complete metric space|complete metric space]]. 

Denote the open ball of radius <math>r > 0</math> in <math>(X, d)</math> by 
<math>B_r := \left\{ p \in X : d(p, \mathbf{0}) < r \right\}</math>
so that when <math>r = 1</math> then 
<math display="block">B_1 = \{ 0 \} \times (-1, 1)</math>
is the open sub-interval of the <math>y</math>-axis strictly between <math>y = -1</math> and <math>y = 1.</math> 
The unit sphere in <math>(X, d)</math> ("unit" meaning that its radius is <math>r = 1</math>) is 
<math display="block">\left\{ p \in X : d(p, \mathbf{0}) = 1 \right\} = S^1</math>
while the closed unit ball in <math>(X, d)</math> is the union of the open unit ball and the unit sphere centered at this same point: 
<math display="block">\left\{ p \in X : d(p, \mathbf{0}) \leq 1 \right\} = S^1 \cup \left(\{ 0 \} \times [-1, 1]\right).</math>

However, the topological boundary <math>\partial_X B_1</math> and topological closure <math>\operatorname{cl}_X B_1</math> in <math>X</math> of the open unit ball <math>B_1</math> are:
<math display="block">\partial_X B_1 = \{ (0, 1), (0, -1) \} \quad \text{ and } \quad \operatorname{cl}_X B_1 ~=~ B_1 \cup \partial_X B_1 ~=~ B_1 \cup\{ (0, 1), (0, -1) \} ~=~\{ 0 \} \times [-1, 1].</math>
In particular, the open unit ball's topological boundary <math>\partial_X B_1 = \{ (0, 1), (0, -1) \}</math> is a {{em|proper}} subset of the unit sphere <math>\left\{ p \in X : d(p, \mathbf{0}) = 1 \right\} = S^1</math> in <math>(X, d).</math> 
And the open unit ball's topological closure <math>\operatorname{cl}_X B_1 = B_1 \cup \{ (0, 1), (0, -1) \}</math> is a proper subset of the closed unit ball <math>\left\{ p \in X : d(p, \mathbf{0}) \leq 1 \right\} = S^1 \cup \left(\{ 0 \} \times [-1, 1]\right)</math> in <math>(X, d).</math> 
The point <math>(1, 0) \in X,</math> for instance, cannot belong to <math>\operatorname{cl}_X B_1</math> because there does not exist a sequence in <math>B_1 = \{ 0 \} \times (-1, 1)</math> that converges to it; the same reasoning generalizes to also explain why no point in <math>X</math> outside of the closed sub-interval <math>\{ 0 \} \times [-1, 1]</math> belongs to <math>\operatorname{cl}_X B_1.</math> Because the topological boundary of the set <math>B_1</math> is always a subset of <math>B_1</math>'s closure, it follows that <math>\partial_X B_1</math> must also be a subset of <math>\{ 0 \} \times [-1, 1].</math> 

In any metric space <math>(M, \rho),</math> the topological boundary in <math>M</math> of an open ball of radius <math>r > 0</math> centered at a point <math>c \in M</math> is always a subset of the sphere of radius <math>r</math> centered at that same point <math>c</math>; that is, 
<math display="block">\partial_M \left(\left\{ m \in M : \rho(m, c) < r \right\}\right) ~\subseteq~ \left\{ m \in M : \rho(m, c)= r \right\}</math>
always holds. 

Moreover, the unit sphere in <math>(X, d)</math> contains <math>X \setminus Y = S^1 \setminus \{ (0, \pm 1) \},</math> which is an open subset of <math>X.</math><ref group="proof">The <math>y</math>-axis <math>Y = \{ 0 \} \times \R</math> is closed in <math>\R^2</math> because it is a product of two closed subsets of <math>\R.</math> Consequently, <math>\R^2 \setminus Y</math> is an open subset of <math>\R^2.</math> Because <math>X</math> has the subspace topology induced by <math>\R^2,</math> the intersection <math>X \cap \left(\R^2 \setminus Y\right) = X \setminus Y</math> is an open subset of <math>X.</math> <math>\blacksquare</math></ref> This shows, in particular, that the unit sphere <math>\left\{ p \in X : d(p, \mathbf{0}) = 1 \right\}</math> in <math>(X, d)</math> contains a {{em|non-empty open}} subset of <math>X.</math>

== Boundary of a boundary ==

For any set <math>S, \partial S \supseteq \partial\partial S,</math> where [[List of mathematical symbols by subject#Set relations|<math>\,\supseteq\,</math>]] denotes the superset with equality holding if and only if the boundary of <math>S</math> has no interior points, which will be the case for example if <math>S</math> is either closed or open.  Since the boundary of a set is closed, <math>\partial \partial S = \partial \partial \partial S</math> for any set <math>S.</math>  The boundary operator thus satisfies a weakened kind of [[Idempotence|idempotence]].

In discussing boundaries of [[Manifold|manifold]]s or [[Simplex|simplex]]es and their [[Simplicial complex|simplicial complex]]es, one often meets the assertion that the boundary of the boundary is always empty.  Indeed, the construction of the [[Singular homology|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 also ==

* See the discussion of boundary in [[Topological manifold|topological manifold]] for more details.
* {{annotated link|Bounding point}}
* {{annotated link|Closure (topology)}}
* [[Interior (topology)#Exterior of a set|Exterior (topology)]] {{en dash}} Largest open set disjoint from some given set
* {{annotated link|Interior (topology)}}
* {{annotated link|Nowhere dense set}}
* {{annotated link|Lebesgue's density theorem}}, for measure-theoretic characterization and properties of boundary
* {{annotated link|Surface (topology)}}

== Notes ==

{{reflist|group=note}}
{{reflist|group=proof}}

== Citations ==

{{reflist}}

== References ==

* {{cite book |last=Munkres |first=J. R. |date=2000 |title=Topology |publisher=Prentice-Hall |isbn=0-13-181629-2}}
* {{cite book |last=Willard |first=S. |date=1970 |title=General Topology |publisher=Addison-Wesley |isbn=0-201-08707-3 |url-access=registration |url=https://archive.org/details/generaltopology00will_0 }}
* {{cite book |last=van den Dries |first= L. |date=1998 |title=Tame Topology |isbn=978-0521598385}}

{{Topology|expanded}}

[[Category:General topology]]

{{Sourceattribution|Boundary (topology)}}