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 of points, along with a set of neighbourhoods for each point, satisfying a set of axioms relating points and neighbourhoods. A topological space is the most general type of a mathematical space that allows for the definition of limits, continuity, and connectedness. Common types of topological spaces include Euclidean spaces, metric spaces and manifolds. Although very general, topological spaces are a fundamental concept used  in virtually every branch of modern mathematics. The branch of mathematics that studies topological spaces in their own right is called point-set topology or general topology. History. Around 1735, Leonhard Euler discovered the formula formula_1 relating the number of vertices, edges and faces of a convex polyhedron, and hence of a planar graph. The study and generalization of this formula, specifically by Cauchy and L'Huilier, is at the origin of topology. In 1827, Carl Friedrich Gauss published "General investigations of curved surfaces" which in section 3 defines the curved surface in a similar manner to the modern topological understanding : "A curved surface is said to possess continuous curvature at one of its points A, if the direction of all the straight lines drawn from A to points of the surface at an infinitely small distance from A are deflected infinitely little from one and the same plane passing through A." Yet, "until Riemann’s work in the early 1850s, surfaces were always dealt with from a local point of view (as parametric surfaces) and topological issues were never considered." "Möbius and Jordan seem to be the first to realize that the main problem about the topology of (compact) surfaces  is to find invariants (preferably numerical) to decide the equivalence of surfaces, that is, to decide whether two surfaces are homeomorphic or not." The subject is clearly defined by Felix Klein in his "Erlangen Program" (1872): the geometry invariants of arbitrary continuous transformation, a kind of geometry. The term "topology" was introduced by Johann Benedict Listing in 1847, although he had used the term in correspondence some years earlier instead of previously used "Analysis situs". The foundation of this science, for a space of any dimension, was created by Henri Poincaré. His first article on this topic appeared in 1894 . In the 1930s, James Waddell Alexander II and Hassler Whitney first expressed the idea that a surface is a topological space that is locally like a Euclidean plane. Topological spaces were first defined by Felix Hausdorff in 1914 in his seminal "Principles of Set Theory". Metric spaces had been defined earlier in 1906 by Maurice Fréchet, though it was Hausdorff who introduced the term "metric space". Definitions. The utility of the notion of a topology is shown by the fact that there are several equivalent definitions of this structure. Thus one chooses the axiomatization suited for the application. The most  commonly used is that in terms of , but perhaps more intuitive is that in terms of and so this is given first. Definition via neighbourhoods. This axiomatization is due to Felix Hausdorff. Let formula_2 be a set; the elements of formula_2 are usually called , though they can be any mathematical object. We allow formula_2 to be empty. Let formula_5 be a function assigning to each formula_6 (point) in formula_2 a non-empty collection formula_8 of subsets of formula_9 The elements of formula_8 will be called of formula_6 with respect to formula_5 (or, simply, ). The function formula_5 is called a neighbourhood  topology if the axioms below are satisfied; and then formula_2 with formula_5 is called a topological space. The first three axioms for neighbourhoods have a clear meaning. The fourth axiom has a very important use in the structure of the theory, that of linking together the neighbourhoods of different points of formula_9 A standard example of such a system of neighbourhoods is for the real line formula_36 where a subset formula_16 of formula_38 is defined to be a of a real number formula_6 if it includes an open interval containing formula_24 Given such a structure, a subset formula_41 of formula_2  is defined to be open if formula_41 is a neighbourhood of all points in formula_44 The open sets then satisfy the axioms given below. Conversely, when given the open sets of a topological space, the neighbourhoods satisfying the above axioms can be recovered by defining formula_16 to be a neighbourhood of formula_6 if formula_16 includes an open set formula_41 such that formula_49 Definition via open sets. A is an ordered pair formula_50 where formula_2 is a set and formula_52 is a collection of subsets of formula_53 satisfying the following axioms: The elements of formula_52 are called open sets and the  collection formula_52 is called a topology on formula_9 A subset formula_63 is said to be in formula_64 if and only if its complement formula_65 is an element of formula_55 Definition via closed sets. Using de Morgan's laws, the above axioms defining open sets become axioms defining closed sets: Using these axioms, another way to define a topological space is as a set formula_2 together with a collection formula_52 of closed subsets of formula_9 Thus the sets in the topology formula_52 are the closed sets, and their complements in formula_2 are the open sets. Other definitions. There are many other  equivalent ways to define a topological space: in other words the concepts of neighbourhood, or that of open or closed sets can be reconstructed from other starting points and satisfy the correct axioms. Another way to define a topological space is by using the Kuratowski closure axioms, which define the closed sets as the fixed points of an operator on the power set of formula_9 A net is a generalisation of the concept of sequence. A topology is completely determined if for every net in formula_2 the set of its accumulation points is specified. Comparison of topologies. A variety of  topologies can be placed on a set to form a topological space. When every set in a topology formula_95 is also in a topology formula_96 and formula_95 is a subset of formula_98 we say that formula_96is than formula_100 and formula_95 is than formula_102 A proof that relies only on the existence of certain open sets will also hold for any finer topology, and similarly a proof that relies only on certain sets not being open applies to any coarser topology. The terms and are sometimes used in place of finer and coarser, respectively. The terms and are also used in  the literature, but with little agreement on the meaning, so one should always be sure of an author's convention when reading. The collection of all topologies on a given fixed set formula_2 forms a complete lattice: if formula_104 is a collection of topologies on formula_53 then the meet of formula_106 is the intersection of formula_107 and the join of formula_106 is the meet of the collection of all topologies on formula_2 that contain every member of formula_110 Continuous functions. A function formula_111 between topological spaces is called continuous if for every formula_112 and every neighbourhood formula_16 of formula_114 there  is a neighbourhood formula_31 of formula_6 such that formula_117 This relates easily to the usual definition in analysis. Equivalently, formula_118 is continuous if the inverse image of every open set is open. This is an attempt to capture the intuition that there are no "jumps" or "separations" in the function. A homeomorphism is a bijection that is continuous and whose inverse is also continuous. Two spaces are called if there exists a homeomorphism between them. From the standpoint of topology, homeomorphic spaces are essentially identical. In category theory, one of the fundamental categories is Top, which denotes the category of  topological spaces whose objects are topological spaces and whose morphisms are continuous functions. The attempt to classify the objects of this category (up to homeomorphism) by invariants has motivated areas of research, such as homotopy theory, homology theory, and K-theory. Examples of topological spaces. A given set may have many different topologies. If a set is given a different topology, it is viewed as a different topological space. Any set can be given the discrete topology in which every subset is open. The only convergent sequences or nets in this topology are those that are eventually constant. Also, any  set can be given the trivial topology (also called the indiscrete topology), in which only the empty set and the whole space are open. Every sequence and net in this topology converges to every point of the space. This example shows that in general topological spaces, limits of sequences need not be unique. However, often topological spaces must be Hausdorff spaces where limit points are unique. Metric spaces. Metric spaces embody a metric, a precise notion of distance between points. Every metric space can be given a metric topology, in which the basic open sets are open balls defined by  the metric. This is the standard topology on any normed vector space. On a finite-dimensional vector space this topology is the same for all norms. There are many ways of defining a topology on formula_36 the set of real numbers. The standard topology on formula_38 is generated by the open intervals. The set of all open intervals forms a base or basis for the topology, meaning that every open set is a union of some collection of sets from the base. In particular, this means that a set is open if there exists an open interval of non zero  radius about every point in the set. More generally, the Euclidean spaces formula_121 can be given a topology. In the usual topology on formula_121 the basic open sets are the open balls. Similarly, formula_123 the set of complex numbers, and formula_124 have a standard topology in which the basic open sets are open balls. Proximity spaces. Proximity spaces provide a notion of closeness of two sets. Uniform spaces. Uniform spaces axiomatize ordering the distance between distinct points. Function spaces. A topological space in which the are functions is called a function space. Cauchy spaces. Cauchy spaces axiomatize the ability to  test whether a net is Cauchy. Cauchy spaces provide a general setting for studying completions. Convergence spaces. Convergence spaces capture some of the features of convergence of filters. Grothendieck sites. Grothendieck sites are categories with additional data axiomatizing whether a family of arrows covers an object. Sites are a general setting for defining sheaves. Other spaces. If formula_125 is a filter on a set formula_2 then formula_127 is a topology on formula_9 Many sets of linear operators in functional analysis are endowed with topologies that are defined by specifying when a particular sequence of functions converges to the zero function . Any local field has a topology native to it, and this can be extended to vector spaces over that field. Every manifold has a natural topology since it is locally Euclidean. Similarly, every simplex and every simplicial complex inherits a natural topology from . The Zariski topology is defined algebraically on the spectrum of a ring or an algebraic variety. On formula_121 or formula_130 the closed sets of the Zariski topology are the solution sets of systems of polynomial equations. A linear graph has a natural topology that generalizes many of the geometric aspects of graphs with vertices and edges. The  Sierpiński space is the simplest non-discrete topological space. It has important relations to the theory of computation and semantics. There exist numerous topologies on any given finite set. Such spaces are called finite topological spaces. Finite spaces are sometimes used to provide examples or counterexamples to conjectures about topological spaces in general. Any set can be given the cofinite topology in which the open sets are the empty set and the sets whose complement is finite. This is the smallest T topology on any infinite set. Any set can be given the cocountable topology, in which a set is  defined as open if it is either empty or its complement is countable. When the set is uncountable, this topology serves as a counterexample in many situations. The real line can also be given the lower limit topology. Here, the basic open sets are the half open intervals formula_131 This topology on formula_38 is strictly finer than the Euclidean topology defined above; a sequence converges to a point in this topology if and only if it converges from above in the Euclidean topology. This example shows that a set may have many distinct topologies defined on it. If formula_125 is  an ordinal number, then the set formula_134 may be endowed with the order topology generated by the intervals formula_135 formula_136 and formula_137 where formula_138 and formula_139 are elements of formula_140 Outer space of a free group formula_141 consists of the so-called "marked metric graph structures" of volume 1 on formula_142 Topological constructions. Every subset of a topological space can be given the subspace topology in which the open sets are the intersections of the open sets of the larger space with the subset. For any indexed family of topological spaces, the product can be given the product topology, which  is generated by the inverse images of open sets of the factors under the projection mappings. For example, in finite products, a basis for the product topology consists of all products of open sets. For infinite products, there is the additional requirement that in a basic open set, all but finitely many of its projections are the entire space. A quotient space is defined as follows: if formula_2 is a topological space and formula_144 is a set, and if formula_111 is a surjective function, then the quotient topology on formula_144 is the collection of subsets of formula_144 that have open  inverse images under formula_148 In other words, the quotient topology is the finest topology on formula_144 for which formula_118 is continuous. A common example of a quotient topology is when an equivalence relation is defined on the topological space formula_9 The map formula_118 is then the natural projection onto the set of equivalence classes. The Vietoris topology on the set of all non-empty subsets of a topological space formula_53 named for Leopold Vietoris, is generated by the following basis: for every formula_154-tuple formula_155 of open sets in formula_53 we construct a basis set consisting of all subsets of  the union of the formula_157 that have non-empty intersections with each formula_158 The Fell topology on the set of all non-empty closed subsets of a locally compact Polish space formula_2 is a variant of the Vietoris topology, and is named after mathematician James Fell. It is generated by the following basis: for every formula_154-tuple formula_155 of open sets in formula_2 and for every compact set formula_163 the set of all subsets of formula_2 that are disjoint from formula_165 and have nonempty intersections with each formula_157 is a member of the basis. Classification of topological spaces. Topological spaces  can be broadly classified, up to homeomorphism, by their topological properties. A topological property is a property of spaces that is invariant under homeomorphisms. To prove that two spaces are not homeomorphic it is sufficient to find a topological property not shared by them. Examples of such properties include connectedness, compactness, and various separation axioms. For algebraic invariants see algebraic topology. Topological spaces with algebraic structure. For any algebraic objects we can introduce the discrete topology, under which the algebraic operations are continuous functions. For any such structure that is not finite, we often have a natural topology compatible with  the algebraic operations, in the sense that the algebraic operations are still continuous. This leads to concepts such as topological groups, topological vector spaces, topological rings and local fields.