In topology and related branches of mathematics, a connected space is a topological space that cannot be represented as the union of two or more disjoint nonempty open subsets. Connectedness is one of the principal topological properties that are used to distinguish topological spaces.
A subset of a topological space is a connected set if it is a connected space when viewed as a subspace of .
Some related but stronger conditions are path connected, simply connected, and connected. Another related notion is locally connected, which neither implies nor follows from connectedness.
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connected space vs disconnected space with examples

Lecture 46  Connected and Disconnected spaces  Topology by James R munkre

Connected Spaces

Topology  Connectedness  Connected Space  Disconnected Space  Theorems

Separated sets  Disconnected space  Connected space
Transcription
Formal definition
A topological space is said to be disconnected if it is the union of two disjoint nonempty open sets. Otherwise, is said to be connected. A subset of a topological space is said to be connected if it is connected under its subspace topology. Some authors exclude the empty set (with its unique topology) as a connected space, but this article does not follow that practice.
For a topological space the following conditions are equivalent:
 is connected, that is, it cannot be divided into two disjoint nonempty open sets.
 The only subsets of which are both open and closed (clopen sets) are and the empty set.
 The only subsets of with empty boundary are and the empty set.
 cannot be written as the union of two nonempty separated sets (sets for which each is disjoint from the other's closure).
 All continuous functions from to are constant, where is the twopoint space endowed with the discrete topology.
Historically this modern formulation of the notion of connectedness (in terms of no partition of into two separated sets) first appeared (independently) with N.J. Lennes, Frigyes Riesz, and Felix Hausdorff at the beginning of the 20th century. See ^{[1]} for details.
Connected components
Given some point in a topological space the union of any collection of connected subsets such that each contains will once again be a connected subset. The connected component of a point in is the union of all connected subsets of that contain it is the unique largest (with respect to ) connected subset of that contains The maximal connected subsets (ordered by inclusion ) of a nonempty topological space are called the connected components of the space. The components of any topological space form a partition of : they are disjoint, nonempty and their union is the whole space. Every component is a closed subset of the original space. It follows that, in the case where their number is finite, each component is also an open subset. However, if their number is infinite, this might not be the case; for instance, the connected components of the set of the rational numbers are the onepoint sets (singletons), which are not open. Proof: Any two distinct rational numbers are in different components. Take an irrational number and then set and Then is a separation of and . Thus each component is a onepoint set.
Let be the connected component of in a topological space and be the intersection of all clopen sets containing (called quasicomponent of ) Then where the equality holds if is compact Hausdorff or locally connected. ^{[2]}
Disconnected spaces
A space in which all components are onepoint sets is called totally disconnected. Related to this property, a space is called totally separated if, for any two distinct elements and of , there exist disjoint open sets containing and containing such that is the union of and . Clearly, any totally separated space is totally disconnected, but the converse does not hold. For example take two copies of the rational numbers , and identify them at every point except zero. The resulting space, with the quotient topology, is totally disconnected. However, by considering the two copies of zero, one sees that the space is not totally separated. In fact, it is not even Hausdorff, and the condition of being totally separated is strictly stronger than the condition of being Hausdorff.
Examples
 The closed interval in the standard subspace topology is connected; although it can, for example, be written as the union of and the second set is not open in the chosen topology of
 The union of and is disconnected; both of these intervals are open in the standard topological space
 is disconnected.
 A convex subset of is connected; it is actually simply connected.
 A Euclidean plane excluding the origin, is connected, but is not simply connected. The threedimensional Euclidean space without the origin is connected, and even simply connected. In contrast, the onedimensional Euclidean space without the origin is not connected.
 A Euclidean plane with a straight line removed is not connected since it consists of two halfplanes.
 , the space of real numbers with the usual topology, is connected.
 The Sorgenfrey line is disconnected.^{[3]}
 If even a single point is removed from , the remainder is disconnected. However, if even a countable infinity of points are removed from , where the remainder is connected. If , then remains simply connected after removal of countably many points.
 Any topological vector space, e.g. any Hilbert space or Banach space, over a connected field (such as or ), is simply connected.
 Every discrete topological space with at least two elements is disconnected, in fact such a space is totally disconnected. The simplest example is the discrete twopoint space.^{[4]}
 On the other hand, a finite set might be connected. For example, the spectrum of a discrete valuation ring consists of two points and is connected. It is an example of a Sierpiński space.
 The Cantor set is totally disconnected; since the set contains uncountably many points, it has uncountably many components.
 If a space is homotopy equivalent to a connected space, then is itself connected.
 The topologist's sine curve is an example of a set that is connected but is neither path connected nor locally connected.
 The general linear group (that is, the group of by real, invertible matrices) consists of two connected components: the one with matrices of positive determinant and the other of negative determinant. In particular, it is not connected. In contrast, is connected. More generally, the set of invertible bounded operators on a complex Hilbert space is connected.
 The spectra of commutative local ring and integral domains are connected. More generally, the following are equivalent^{[5]}
 The spectrum of a commutative ring is connected
 Every finitely generated projective module over has constant rank.
 has no idempotent (i.e., is not a product of two rings in a nontrivial way).
An example of a space that is not connected is a plane with an infinite line deleted from it. Other examples of disconnected spaces (that is, spaces which are not connected) include the plane with an annulus removed, as well as the union of two disjoint closed disks, where all examples of this paragraph bear the subspace topology induced by twodimensional Euclidean space.
Path connectedness
A pathconnected space is a stronger notion of connectedness, requiring the structure of a path. A path from a point to a point in a topological space is a continuous function from the unit interval to with and . A pathcomponent of is an equivalence class of under the equivalence relation which makes equivalent to if there is a path from to . The space is said to be pathconnected (or pathwise connected or connected) if there is exactly one pathcomponent, i.e. if there is a path joining any two points in . Again, many authors exclude the empty space (by this definition, however, the empty space is not pathconnected because it has zero pathcomponents; there is a unique equivalence relation on the empty set which has zero equivalence classes).
Every pathconnected space is connected. The converse is not always true: examples of connected spaces that are not pathconnected include the extended long line and the topologist's sine curve.
Subsets of the real line are connected if and only if they are pathconnected; these subsets are the intervals of . Also, open subsets of or are connected if and only if they are pathconnected. Additionally, connectedness and pathconnectedness are the same for finite topological spaces.
Arc connectedness
A space is said to be arcconnected or arcwise connected if any two topologically distinguishable points can be joined by an arc, which is an embedding . An arccomponent of is a maximal arcconnected subset of ; or equivalently an equivalence class of the equivalence relation of whether two points can be joined by an arc or by a path whose points are topologically indistinguishable.
Every Hausdorff space that is pathconnected is also arcconnected; more generally this is true for a Hausdorff space, which is a space where each image of a path is closed. An example of a space which is pathconnected but not arcconnected is given by the line with two origins; its two copies of can be connected by a path but not by an arc.
Intuition for pathconnected spaces does not readily transfer to arcconnected spaces. Let be the line with two origins. The following are facts whose analogues hold for pathconnected spaces, but do not hold for arcconnected spaces:
 Continuous image of arcconnected space may not be arcconnected: for example, a quotient map from an arcconnected space to its quotient with countably many (at least 2) topologically distinguishable points cannot be arcconnected due to too small cardinality.
 Arccomponents may not be disjoint. For example, has two overlapping arccomponents.
 Arcconnected product space may not be a product of arcconnected spaces. For example, is arcconnected, but is not.
 Arccomponents of a product space may not be products of arccomponents of the marginal spaces. For example, has a single arccomponent, but has two arccomponents.
 If arcconnected subsets have a nonempty intersection, then their union may not be arcconnected. For example, the arccomponents of intersect, but their union is not arcconnected.
Local connectedness
A topological space is said to be locally connected at a point if every neighbourhood of contains a connected open neighbourhood. It is locally connected if it has a base of connected sets. It can be shown that a space is locally connected if and only if every component of every open set of is open.
Similarly, a topological space is said to be locally pathconnected if it has a base of pathconnected sets. An open subset of a locally pathconnected space is connected if and only if it is pathconnected. This generalizes the earlier statement about and , each of which is locally pathconnected. More generally, any topological manifold is locally pathconnected.
Locally connected does not imply connected, nor does locally pathconnected imply path connected. A simple example of a locally connected (and locally pathconnected) space that is not connected (or pathconnected) is the union of two separated intervals in , such as .
A classical example of a connected space that is not locally connected is the so called topologist's sine curve, defined as , with the Euclidean topology induced by inclusion in .
Set operations
The intersection of connected sets is not necessarily connected.
The union of connected sets is not necessarily connected, as can be seen by considering .
Each ellipse is a connected set, but the union is not connected, since it can be partitioned to two disjoint open sets and .
This means that, if the union is disconnected, then the collection can be partitioned to two subcollections, such that the unions of the subcollections are disjoint and open in (see picture). This implies that in several cases, a union of connected sets is necessarily connected. In particular:
 If the common intersection of all sets is not empty (), then obviously they cannot be partitioned to collections with disjoint unions. Hence the union of connected sets with nonempty intersection is connected.
 If the intersection of each pair of sets is not empty () then again they cannot be partitioned to collections with disjoint unions, so their union must be connected.
 If the sets can be ordered as a "linked chain", i.e. indexed by integer indices and , then again their union must be connected.
 If the sets are pairwisedisjoint and the quotient space is connected, then X must be connected. Otherwise, if is a separation of X then is a separation of the quotient space (since are disjoint and open in the quotient space).^{[6]}
The set difference of connected sets is not necessarily connected. However, if and their difference is disconnected (and thus can be written as a union of two open sets and ), then the union of with each such component is connected (i.e. is connected for all ).
By contradiction, suppose is not connected. So it can be written as the union of two disjoint open sets, e.g. . Because is connected, it must be entirely contained in one of these components, say , and thus is contained in . Now we know that:
Theorems
 Main theorem of connectedness: Let and be topological spaces and let be a continuous function. If is (path)connected then the image is (path)connected. This result can be considered a generalization of the intermediate value theorem.
 Every pathconnected space is connected.
 Every locally pathconnected space is locally connected.
 A locally pathconnected space is pathconnected if and only if it is connected.
 The closure of a connected subset is connected. Furthermore, any subset between a connected subset and its closure is connected.
 The connected components are always closed (but in general not open)
 The connected components of a locally connected space are also open.
 The connected components of a space are disjoint unions of the pathconnected components (which in general are neither open nor closed).
 Every quotient of a connected (resp. locally connected, pathconnected, locally pathconnected) space is connected (resp. locally connected, pathconnected, locally pathconnected).
 Every product of a family of connected (resp. pathconnected) spaces is connected (resp. pathconnected).
 Every open subset of a locally connected (resp. locally pathconnected) space is locally connected (resp. locally pathconnected).
 Every manifold is locally pathconnected.
 Arcwise connected space is path connected, but pathwise connected space may not be arcwise connected
 Continuous image of arcwise connected set is arcwise connected.
Graphs
Graphs have path connected subsets, namely those subsets for which every pair of points has a path of edges joining them. But it is not always possible to find a topology on the set of points which induces the same connected sets. The 5cycle graph (and any cycle with odd) is one such example.
As a consequence, a notion of connectedness can be formulated independently of the topology on a space. To wit, there is a category of connective spaces consisting of sets with collections of connected subsets satisfying connectivity axioms; their morphisms are those functions which map connected sets to connected sets (Muscat & Buhagiar 2006). Topological spaces and graphs are special cases of connective spaces; indeed, the finite connective spaces are precisely the finite graphs.
However, every graph can be canonically made into a topological space, by treating vertices as points and edges as copies of the unit interval (see topological graph theory#Graphs as topological spaces). Then one can show that the graph is connected (in the graph theoretical sense) if and only if it is connected as a topological space.
Stronger forms of connectedness
There are stronger forms of connectedness for topological spaces, for instance:
 If there exist no two disjoint nonempty open sets in a topological space , must be connected, and thus hyperconnected spaces are also connected.
 Since a simply connected space is, by definition, also required to be path connected, any simply connected space is also connected. If the "path connectedness" requirement is dropped from the definition of simple connectivity, a simply connected space does not need to be connected.
 Yet stronger versions of connectivity include the notion of a contractible space. Every contractible space is path connected and thus also connected.
In general, any path connected space must be connected but there exist connected spaces that are not path connected. The deleted comb space furnishes such an example, as does the abovementioned topologist's sine curve.
See also
 Connected component (graph theory) – Maximal subgraph whose vertices can reach each other
 Connectedness locus
 Domain (mathematical analysis) – Connected open subset of a topological space
 Extremally disconnected space – Topological space in which the closure of every open set is open
 Locally connected space – Property of topological spaces
 nconnected
 Uniformly connected space – Type of uniform space
 Pixel connectivity
References
 ^ Wilder, R.L. (1978). "Evolution of the Topological Concept of "Connected"". American Mathematical Monthly. 85 (9): 720–726. doi:10.2307/2321676. JSTOR 2321676.
 ^ "General topology  Components of the set of rational numbers".
 ^ Stephen Willard (1970). General Topology. Dover. p. 191. ISBN 0486434796.
 ^ George F. Simmons (1968). Introduction to Topology and Modern Analysis. McGraw Hill Book Company. p. 144. ISBN 0898745519.
 ^ Charles Weibel, The Kbook: An introduction to algebraic Ktheory
 ^ Brandsma, Henno (February 13, 2013). "How to prove this result involving the quotient maps and connectedness?". Stack Exchange.
 ^ Marek (February 13, 2013). "How to prove this result about connectedness?". Stack Exchange.
Further reading
 Munkres, James R. (2000). Topology, Second Edition. Prentice Hall. ISBN 0131816292.
 Weisstein, Eric W. "Connected Set". MathWorld.
 V. I. Malykhin (2001) [1994], "Connected space", Encyclopedia of Mathematics, EMS Press
 Muscat, J; Buhagiar, D (2006). "Connective Spaces" (PDF). Mem. Fac. Sci. Eng. Shimane Univ., Series B: Math. Sc. 39: 1–13. Archived from the original (PDF) on 20160304. Retrieved 20100517..