Topological group

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Topological_group

The real numbers form a topological group under addition

In mathematics, topological groups are the combination of groups and topological spaces, i.e. they are groups and topological spaces at the same time, such that the continuity condition for the group operations connects these two structures together and consequently they are not independent from each other.

Topological groups have been studied extensively in the period of 1925 to 1940. Haar and Weil (respectively in 1933 and 1940) showed that the integrals and Fourier series are special cases of a very wide class of topological groups.

Topological groups, along with continuous group actions, are used to study continuous symmetries, which have many applications, for example, in physics. In functional analysis, every topological vector space is an additive topological group with the additional property that scalar multiplication is continuous; consequently, many results from the theory of topological groups can be applied to functional analysis.

Formal definition

A topological group, G, is a topological space that is also a group such that the group operation (in this case product):

⋅ : G × G → G, (x, y) ↦ xy

and the inversion map:

⁻¹ : G → G, x ↦ x⁻¹

are continuous. Here G × G is viewed as a topological space with the product topology. Such a topology is said to be compatible with the group operations and is called a group topology.

Checking continuity

The product map is continuous if and only if for any x, y ∈ G and any neighborhood W of xy in G, there exist neighborhoods U of x and V of y in G such that U ⋅ V ⊆ W, where U ⋅ V := {u ⋅ v : u ∈ U, v ∈ V}. The inversion map is continuous if and only if for any x ∈ G and any neighborhood V of x⁻¹ in G, there exists a neighborhood U of x in G such that U⁻¹ ⊆ V, where U⁻¹ := { u⁻¹ : u ∈ U }.

To show that a topology is compatible with the group operations, it suffices to check that the map

G × G → G, (x, y) ↦ xy⁻¹

is continuous. Explicitly, this means that for any x, y ∈ G and any neighborhood W in G of xy⁻¹, there exist neighborhoods U of x and V of y in G such that U ⋅ (V⁻¹) ⊆ W.

Additive notation

This definition used notation for multiplicative groups; the equivalent for additive groups would be that the following two operations are continuous:

+ : G × G → G , (x, y) ↦ x + y

− : G → G , x ↦ −x.

Hausdorffness

Although not part of this definition, many authors require that the topology on G be Hausdorff. One reason for this is that any topological group can be canonically associated with a Hausdorff topological group by taking an appropriate canonical quotient; this however, often still requires working with the original non-Hausdorff topological group. Other reasons, and some equivalent conditions, are discussed below.

This article will not assume that topological groups are necessarily Hausdorff.

Category

In the language of category theory, topological groups can be defined concisely as group objects in the category of topological spaces, in the same way that ordinary groups are group objects in the category of sets. Note that the axioms are given in terms of the maps (binary product, unary inverse, and nullary identity), hence are categorical definitions.

Homomorphisms

A homomorphism of topological groups means a continuous group homomorphism G → H. Topological groups, together with their homomorphisms, form a category. A group homomorphism between topological groups is continuous if and only if it is continuous at some point.

An isomorphism of topological groups is a group isomorphism that is also a homeomorphism of the underlying topological spaces. This is stronger than simply requiring a continuous group isomorphism—the inverse must also be continuous. There are examples of topological groups that are isomorphic as ordinary groups but not as topological groups. Indeed, any non-discrete topological group is also a topological group when considered with the discrete topology. The underlying groups are the same, but as topological groups there is not an isomorphism.

Examples

Every group can be trivially made into a topological group by considering it with the discrete topology; such groups are called discrete groups. In this sense, the theory of topological groups subsumes that of ordinary groups. The indiscrete topology (i.e. the trivial topology) also makes every group into a topological group.

The real numbers, ${\displaystyle \mathbb{R}}$ with the usual topology form a topological group under addition. Euclidean n-space ${\displaystyle \mathbb{R}}$ⁿ is also a topological group under addition, and more generally, every topological vector space forms an (abelian) topological group. Some other examples of abelian topological groups are the circle group S¹, or the torus (S¹)ⁿ for any natural number n.

The classical groups are important examples of non-abelian topological groups. For instance, the general linear group GL(n,${\displaystyle \mathbb{R}}$) of all invertible n-by-n matrices with real entries can be viewed as a topological group with the topology defined by viewing GL(n,${\displaystyle \mathbb{R}}$) as a subspace of Euclidean space ${\displaystyle \mathbb{R}}$^n×n. Another classical group is the orthogonal group O(n), the group of all linear maps from ${\displaystyle \mathbb{R}}$ⁿ to itself that preserve the length of all vectors. The orthogonal group is compact as a topological space. Much of Euclidean geometry can be viewed as studying the structure of the orthogonal group, or the closely related group O(n) ⋉ ${\displaystyle \mathbb{R}}$ⁿ of isometries of ${\displaystyle \mathbb{R}}$ⁿ.

The groups mentioned so far are all Lie groups, meaning that they are smooth manifolds in such a way that the group operations are smooth, not just continuous. Lie groups are the best-understood topological groups; many questions about Lie groups can be converted to purely algebraic questions about Lie algebras and then solved.

An example of a topological group that is not a Lie group is the additive group ${\displaystyle \mathbb{Q}}$ of rational numbers, with the topology inherited from ${\displaystyle \mathbb{R}}$. This is a countable space, and it does not have the discrete topology. An important example for number theory is the group ${\displaystyle \mathbb{Z}}$_p of p-adic integers, for a prime number p, meaning the inverse limit of the finite groups ${\displaystyle \mathbb{Z}}$/pⁿ as n goes to infinity. The group ${\displaystyle \mathbb{Z}}$_p is well behaved in that it is compact (in fact, homeomorphic to the Cantor set), but it differs from (real) Lie groups in that it is totally disconnected. More generally, there is a theory of p-adic Lie groups, including compact groups such as GL(n,${\displaystyle \mathbb{Z}}$_p) as well as locally compact groups such as GL(n,${\displaystyle \mathbb{Q}}$_p), where ${\displaystyle \mathbb{Q}}$_p is the locally compact field of p-adic numbers.

The group ${\displaystyle \mathbb{Z}}$_p is a pro-finite group; it is isomorphic to a subgroup of the product ${\displaystyle \prod _{n\ge 1}\mathbb{Z}/p^n}$ in such a way that its topology is induced by the product topology, where the finite groups ${\displaystyle \mathbb{Z}/p^n}$ are given the discrete topology. Another large class of pro-finite groups important in number theory are absolute Galois groups.

Some topological groups can be viewed as infinite dimensional Lie groups; this phrase is best understood informally, to include several different families of examples. For example, a topological vector space, such as a Banach space or Hilbert space, is an abelian topological group under addition. Some other infinite-dimensional groups that have been studied, with varying degrees of success, are loop groups, Kac–Moody groups, diffeomorphism groups, homeomorphism groups, and gauge groups.

In every Banach algebra with multiplicative identity, the set of invertible elements forms a topological group under multiplication. For example, the group of invertible bounded operators on a Hilbert space arises this way.

Properties

Translation invariance

Every topological group's topology is translation invariant, which by definition means that if for any ${\displaystyle a\in G,}$ left or right multiplication by this element yields a homeomorphism ${\displaystyle G\to G.}$ Consequently, for any ${\displaystyle a\in G}$ and ${\displaystyle S\subseteq G,}$ the subset ${\displaystyle S}$ is open (resp. closed) in ${\displaystyle G}$ if and only if this is true of its left translation ${\displaystyle aS:=\{as:s\in S\}}$ and right translation ${\displaystyle Sa:=\{sa:s\in S\}.}$ If ${\displaystyle \mathcal{N}}$ is a neighborhood basis of the identity element in a topological group ${\displaystyle G}$ then for all ${\displaystyle x\in X,}$ ${\displaystyle x\mathcal{N}:=\{xN:N\in \mathcal{N}\}}$ is a neighborhood basis of ${\displaystyle x}$ in ${\displaystyle G.}$ In particular, any group topology on a topological group is completely determined by any neighborhood basis at the identity element. If ${\displaystyle S}$ is any subset of ${\displaystyle G}$ and ${\displaystyle U}$ is an open subset of ${\displaystyle G,}$ then ${\displaystyle SU:=\{su:s\in S,u\in U\}}$ is an open subset of ${\displaystyle G.}$

Symmetric neighborhoods

The inversion operation ${\displaystyle g\mapsto g^{-1}}$ on a topological group ${\displaystyle G}$ is a homeomorphism from ${\displaystyle G}$ to itself.

A subset ${\displaystyle S\subseteq G}$ is said to be symmetric if ${\displaystyle S^{-1}=S,}$ where ${\displaystyle S^{-1}:=\left\{s^{-1}:s\in S\right\}.}$ The closure of every symmetric set in a commutative topological group is symmetric. If S is any subset of a commutative topological group G, then the following sets are also symmetric: S⁻¹ ∩ S, S⁻¹ ∪ S, and S⁻¹ S.

For any neighborhood N in a commutative topological group G of the identity element, there exists a symmetric neighborhood M of the identity element such that M⁻¹ M ⊆ N, where note that M⁻¹ M is necessarily a symmetric neighborhood of the identity element. Thus every topological group has a neighborhood basis at the identity element consisting of symmetric sets.

If G is a locally compact commutative group, then for any neighborhood N in G of the identity element, there exists a symmetric relatively compact neighborhood M of the identity element such that cl M ⊆ N (where cl M is symmetric as well).

Uniform space

Every topological group can be viewed as a uniform space in two ways; the left uniformity turns all left multiplications into uniformly continuous maps while the right uniformity turns all right multiplications into uniformly continuous maps. If G is not abelian, then these two need not coincide. The uniform structures allow one to talk about notions such as completeness, uniform continuity and uniform convergence on topological groups.

Separation properties

If U is an open subset of a commutative topological group G and U contains a compact set K, then there exists a neighborhood N of the identity element such that KN ⊆ U.

As a uniform space, every commutative topological group is completely regular. Consequently, for a multiplicative topological group G with identity element 1, the following are equivalent:

1. G is a T₀-space (Kolmogorov);
2. G is a T₂-space (Hausdorff);
3. G is a T_31⁄2 (Tychonoff);
4. { 1 } is closed in G;
5. { 1 } := ∩N ∈ 𝒩 N, where 𝒩 is a neighborhood basis of the identity element in G;
6. for any ${\displaystyle x\in G}$ such that ${\displaystyle x\ne 1,}$ there exists a neighborhood U in G of the identity element such that ${\displaystyle x\notin U.}$

A subgroup of a commutative topological group is discrete if and only if it has an isolated point.

If G is not Hausdorff, then one can obtain a Hausdorff group by passing to the quotient group G/K, where K is the closure of the identity. This is equivalent to taking the Kolmogorov quotient of G.

Metrisability

Let G be a topological group. As with any topological space, we say that G is metrisable if and only if there exists a metric d on G, which induces the same topology on ${\displaystyle G}$. A metric d on G is called

• left-invariant (resp. right-invariant) if and only if ${\displaystyle d(a{x}_1,a{x}_2)=d(x_1,x_2)}$(resp. ${\displaystyle d(x_{1}a,x_{2}a)=d(x_1,x_2)}$) for all ${\displaystyle a,x_1,x_2\in G}$ (equivalently, ${\displaystyle d}$ is left-invariant just in case the map ${\displaystyle x\mapsto ax}$ is an isometry from ${\displaystyle (G,d)}$ to itself for each ${\displaystyle a\in G}$).
• proper if and only if all open balls, ${\displaystyle B(r)=\{g\in G\mid d(g,1)<r\}}$ for ${\displaystyle r>0}$, are pre-compact.

The Birkhoff–Kakutani theorem (named after mathematicians Garrett Birkhoff and Shizuo Kakutani) states that the following three conditions on a topological group G are equivalent:

1. G is first countable (equivalently: the identity element 1 is closed in G, and there is a countable basis of neighborhoods for 1 in G).
2. G is metrisable (as a topological space).
3. There is a left-invariant metric on G that induces the given topology on G.

Furthermore, the following are equivalent for any topological group G:

1. G is a second countable locally compact (Hausdorff) space.
2. G is a Polish, locally compact (Hausdorff) space.
3. G is properly metrisable (as a topological space).
4. There is a left-invariant, proper metric on G that induces the given topology on G.

Note: As with the rest of the article we of assume here a Hausdorff topology. The implications 4 ${\displaystyle \Rightarrow }$ 3 ${\displaystyle \Rightarrow }$ 2 ${\displaystyle \Rightarrow }$ 1 hold in any topological space. In particular 3 ${\displaystyle \Rightarrow }$ 2 holds, since in particular any properly metrisable space is countable union of compact metrisable and thus separable (cf. properties of compact metric spaces) subsets. The non-trivial implication 1 ${\displaystyle \Rightarrow }$ 4 was first proved by Raimond Struble in 1974. An alternative approach was made by Uffe Haagerup and Agata Przybyszewska in 2006, the idea of the which is as follows: One relies on the construction of a left-invariant metric, ${\displaystyle d_0}$, as in the case of first countable spaces. By local compactness, closed balls of sufficiently small radii are compact, and by normalising we can assume this holds for radius 1. Closing the open ball, U, of radius 1 under multiplication yields a clopen subgroup, H, of G, on which the metric ${\displaystyle d_0}$ is proper. Since H is open and G is second countable, the subgroup has at most countably many cosets. One now uses this sequence of cosets and the metric on H to construct a proper metric on G.

Subgroups

Every subgroup of a topological group is itself a topological group when given the subspace topology. Every open subgroup H is also closed in G, since the complement of H is the open set given by the union of open sets gH for g ∈ G \ H. If H is a subgroup of G then the closure of H is also a subgroup. Likewise, if H is a normal subgroup of G, the closure of H is normal in G.

Quotients and normal subgroups

If H is a subgroup of G, the set of left cosets G/H with the quotient topology is called a homogeneous space for G. The quotient map ${\displaystyle q:G\to G/H}$ is always open. For example, for a positive integer n, the sphere Sⁿ is a homogeneous space for the rotation group SO(n+1) in ${\displaystyle \mathbb{R}}$ⁿ⁺¹, with Sⁿ = SO(n+1)/SO(n). A homogeneous space G/H is Hausdorff if and only if H is closed in G. Partly for this reason, it is natural to concentrate on closed subgroups when studying topological groups.

If H is a normal subgroup of G, then the quotient group G/H becomes a topological group when given the quotient topology. It is Hausdorff if and only if H is closed in G. For example, the quotient group ${\displaystyle \mathbb{R}/\mathbb{Z}}$ is isomorphic to the circle group S¹.

In any topological group, the identity component (i.e., the connected component containing the identity element) is a closed normal subgroup. If C is the identity component and a is any point of G, then the left coset aC is the component of G containing a. So the collection of all left cosets (or right cosets) of C in G is equal to the collection of all components of G. It follows that the quotient group G/C is totally disconnected.

Closure and compactness

In any commutative topological group, the product (assuming the group is multiplicative) KC of a compact set K and a closed set C is a closed set. Furthermore, for any subsets R and S of G, (cl R)(cl S) ⊆ cl (RS).

If H is a subgroup of a commutative topological group G and if N is a neighborhood in G of the identity element such that H ∩ cl N is closed, then H is closed. Every discrete subgroup of a Hausdorff commutative topological group is closed.

Isomorphism theorems

The isomorphism theorems from ordinary group theory are not always true in the topological setting. This is because a bijective homomorphism need not be an isomorphism of topological groups.

For example, a native version of the first isomorphism theorem is false for topological groups: if ${\displaystyle f:G\to H}$ is a morphism of topological groups (that is, a continuous homomorphism), it is not necessarily true that the induced homomorphism ${\displaystyle \tilde{f}:G/\ker f\to \mathrm{I}\mathrm{m}(f)}$ is an isomorphism of topological groups; it will be a bijective, continuous homomorphism, but it will not necessarily be a homeomorphism. In other words, it will not necessarily admit an inverse in the category of topological groups.

There is a version of the first isomorphism theorem for topological groups, which may be stated as follows: if ${\displaystyle f:G\to H}$ is a continuous homomorphism, then the induced homomorphism from G/ker(f) to im(f) is an isomorphism if and only if the map f is open onto its image.

The third isomorphism theorem, however, is true more or less verbatim for topological groups, as one may easily check.

Hilbert's fifth problem

There are several strong results on the relation between topological groups and Lie groups. First, every continuous homomorphism of Lie groups ${\displaystyle G\to H}$ is smooth. It follows that a topological group has a unique structure of a Lie group if one exists. Also, Cartan's theorem says that every closed subgroup of a Lie group is a Lie subgroup, in particular a smooth submanifold.

Hilbert's fifth problem asked whether a topological group G that is a topological manifold must be a Lie group. In other words, does G have the structure of a smooth manifold, making the group operations smooth? As shown by Andrew Gleason, Deane Montgomery, and Leo Zippin, the answer to this problem is yes. In fact, G has a real analytic structure. Using the smooth structure, one can define the Lie algebra of G, an object of linear algebra that determines a connected group G up to covering spaces. As a result, the solution to Hilbert's fifth problem reduces the classification of topological groups that are topological manifolds to an algebraic problem, albeit a complicated problem in general.

The theorem also has consequences for broader classes of topological groups. First, every compact group (understood to be Hausdorff) is an inverse limit of compact Lie groups. (One important case is an inverse limit of finite groups, called a profinite group. For example, the group ${\displaystyle \mathbb{Z}}$_p of p-adic integers and the absolute Galois group of a field are profinite groups.) Furthermore, every connected locally compact group is an inverse limit of connected Lie groups. At the other extreme, a totally disconnected locally compact group always contains a compact open subgroup, which is necessarily a profinite group. (For example, the locally compact group GL(n,${\displaystyle \mathbb{Q}}$_p) contains the compact open subgroup GL(n,${\displaystyle \mathbb{Z}}$_p), which is the inverse limit of the finite groups GL(n,${\displaystyle \mathbb{Z}}$/p^r) as r' goes to infinity.)

Representations of compact or locally compact groups

An action of a topological group G on a topological space X is a group action of G on X such that the corresponding function G × X → X is continuous. Likewise, a representation of a topological group G on a real or complex topological vector space V is a continuous action of G on V such that for each g ∈ G, the map v ↦ gv from V to itself is linear.

Group actions and representation theory are particularly well understood for compact groups, generalizing what happens for finite groups. For example, every finite-dimensional (real or complex) representation of a compact group is a direct sum of irreducible representations. An infinite-dimensional unitary representation of a compact group can be decomposed as a Hilbert-space direct sum of irreducible representations, which are all finite-dimensional; this is part of the Peter–Weyl theorem. For example, the theory of Fourier series describes the decomposition of the unitary representation of the circle group S¹ on the complex Hilbert space L²(S¹). The irreducible representations of S¹ are all 1-dimensional, of the form z ↦ zⁿ for integers n (where S¹ is viewed as a subgroup of the multiplicative group ${\displaystyle \mathbb{C}}$*). Each of these representations occurs with multiplicity 1 in L²(S¹).

The irreducible representations of all compact connected Lie groups have been classified. In particular, the character of each irreducible representation is given by the Weyl character formula.

More generally, locally compact groups have a rich theory of harmonic analysis, because they admit a natural notion of measure and integral, given by the Haar measure. Every unitary representation of a locally compact group can be described as a direct integral of irreducible unitary representations. (The decomposition is essentially unique if G is of Type I, which includes the most important examples such as abelian groups and semisimple Lie groups.) A basic example is the Fourier transform, which decomposes the action of the additive group ${\displaystyle \mathbb{R}}$ on the Hilbert space L²(${\displaystyle \mathbb{R}}$) as a direct integral of the irreducible unitary representations of ${\displaystyle \mathbb{R}}$. The irreducible unitary representations of ${\displaystyle \mathbb{R}}$ are all 1-dimensional, of the form x ↦ e^2πiax for a ∈ ${\displaystyle \mathbb{R}}$.

The irreducible unitary representations of a locally compact group may be infinite-dimensional. A major goal of representation theory, related to the Langlands classification of admissible representations, is to find the unitary dual (the space of all irreducible unitary representations) for the semisimple Lie groups. The unitary dual is known in many cases such as SL(2,${\displaystyle \mathbb{R}}$), but not all.

For a locally compact abelian group G, every irreducible unitary representation has dimension 1. In this case, the unitary dual ${\displaystyle \hat{G}}$ is a group, in fact another locally compact abelian group. Pontryagin duality states that for a locally compact abelian group G, the dual of ${\displaystyle \hat{G}}$ is the original group G. For example, the dual group of the integers ${\displaystyle \mathbb{Z}}$ is the circle group S¹, while the group ${\displaystyle \mathbb{R}}$ of real numbers is isomorphic to its own dual.

Every locally compact group G has a good supply of irreducible unitary representations; for example, enough representations to distinguish the points of G (the Gelfand–Raikov theorem). By contrast, representation theory for topological groups that are not locally compact has so far been developed only in special situations, and it may not be reasonable to expect a general theory. For example, there are many abelian Banach–Lie groups for which every representation on Hilbert space is trivial.

Homotopy theory of topological groups

Topological groups are special among all topological spaces, even in terms of their homotopy type. One basic point is that a topological group G determines a path-connected topological space, the classifying space BG (which classifies principal G-bundles over topological spaces, under mild hypotheses). The group G is isomorphic in the homotopy category to the loop space of BG; that implies various restrictions on the homotopy type of G. Some of these restrictions hold in the broader context of H-spaces.

For example, the fundamental group of a topological group G is abelian. (More generally, the Whitehead product on the homotopy groups of G is zero.) Also, for any field k, the cohomology ring H*(G,k) has the structure of a Hopf algebra. In view of structure theorems on Hopf algebras by Heinz Hopf and Armand Borel, this puts strong restrictions on the possible cohomology rings of topological groups. In particular, if G is a path-connected topological group whose rational cohomology ring H*(G,${\displaystyle \mathbb{Q}}$) is finite-dimensional in each degree, then this ring must be a free graded-commutative algebra over ${\displaystyle \mathbb{Q}}$, that is, the tensor product of a polynomial ring on generators of even degree with an exterior algebra on generators of odd degree.

In particular, for a connected Lie group G, the rational cohomology ring of G is an exterior algebra on generators of odd degree. Moreover, a connected Lie group G has a maximal compact subgroup K, which is unique up to conjugation, and the inclusion of K into G is a homotopy equivalence. So describing the homotopy types of Lie groups reduces to the case of compact Lie groups. For example, the maximal compact subgroup of SL(2,${\displaystyle \mathbb{R}}$) is the circle group SO(2), and the homogeneous space SL(2,${\displaystyle \mathbb{R}}$)/SO(2) can be identified with the hyperbolic plane. Since the hyperbolic plane is contractible, the inclusion of the circle group into SL(2,${\displaystyle \mathbb{R}}$) is a homotopy equivalence.

Finally, compact connected Lie groups have been classified by Wilhelm Killing, Élie Cartan, and Hermann Weyl. As a result, there is an essentially complete description of the possible homotopy types of Lie groups. For example, a compact connected Lie group of dimension at most 3 is either a torus, the group SU(2) (diffeomorphic to the 3-sphere S³), or its quotient group SU(2)/{±1} ≅ SO(3) (diffeomorphic to RP³).

Complete topological group

See also: Complete uniform space

Information about convergence of nets and filters, such as definitions and properties, can be found in the article about filters in topology.

Canonical uniformity on a commutative topological group

Main article: Uniform space

This article will henceforth assume that any topological group that we consider is an additive commutative topological group with identity element ${\displaystyle 0.}$

The diagonal of ${\displaystyle X}$ is the set

$${\displaystyle {\mathrm{\Delta }}_X:=\{(x,x):x\in X\}}$$

and for any ${\displaystyle N\subseteq X}$ containing ${\displaystyle 0,}$ the canonical entourage or canonical vicinities around ${\displaystyle N}$ is the set

$${\displaystyle {\mathrm{\Delta }}_X(N):=\{(x,y)\in X\times X:x-y\in N\}=\bigcup _{y\in X}[(y+N)\times \{y\}]={\mathrm{\Delta }}_X+(N\times \{0\})}$$

For a topological group ${\displaystyle (X,\tau ),}$ the canonical uniformity on ${\displaystyle X}$ is the uniform structure induced by the set of all canonical entourages ${\displaystyle \mathrm{\Delta }(N)}$ as ${\displaystyle N}$ ranges over all neighborhoods of ${\displaystyle 0}$ in ${\displaystyle X.}$

That is, it is the upward closure of the following prefilter on ${\displaystyle X\times X,}$

$${\displaystyle \left\{\mathrm{\Delta }(N):N\text{ is a neighborhood of }0\text{ in }X\right\}}$$

where this prefilter forms what is known as a base of entourages of the canonical uniformity.

For a commutative additive group ${\displaystyle X,}$ a fundamental system of entourages ${\displaystyle \mathcal{B}}$ is called a translation-invariant uniformity if for every ${\displaystyle B\in \mathcal{B},}$ ${\displaystyle (x,y)\in B}$ if and only if ${\displaystyle (x+z,y+z)\in B}$ for all ${\displaystyle x,y,z\in X.}$ A uniformity ${\displaystyle \mathcal{B}}$ is called translation-invariant if it has a base of entourages that is translation-invariant.

• The canonical uniformity on any commutative topological group is translation-invariant.
• The same canonical uniformity would result by using a neighborhood basis of the origin rather the filter of all neighborhoods of the origin.
• Every entourage ${\displaystyle {\mathrm{\Delta }}_X(N)}$ contains the diagonal ${\displaystyle {\mathrm{\Delta }}_X:={\mathrm{\Delta }}_X(\{0\})=\{(x,x):x\in X\}}$ because ${\displaystyle 0\in N.}$
• If ${\displaystyle N}$ is symmetric (that is, ${\displaystyle -N=N}$) then ${\displaystyle {\mathrm{\Delta }}_X(N)}$ is symmetric (meaning that ${\displaystyle {\mathrm{\Delta }}_X(N{)}^{\mathrm{op}}={\mathrm{\Delta }}_X(N)}$) and ${\displaystyle {\mathrm{\Delta }}_X(N)\circ {\mathrm{\Delta }}_X(N)=\{(x,z):\text{ there exists }y\in X\text{ such that }x,z\in y+N\}=\bigcup _{y\in X}[(y+N)\times (y+N)]={\mathrm{\Delta }}_X+(N\times N).}$
• The topology induced on ${\displaystyle X}$ by the canonical uniformity is the same as the topology that ${\displaystyle X}$ started with (that is, it is ${\displaystyle \tau }$).

Cauchy prefilters and nets

Main articles: Filters in topology and Net (mathematics)

The general theory of uniform spaces has its own definition of a "Cauchy prefilter" and "Cauchy net." For the canonical uniformity on ${\displaystyle X,}$ these reduces down to the definition described below.

Suppose ${\displaystyle x_{\bullet }={\left(x_i\right)}_{i\in I}}$ is a net in ${\displaystyle X}$ and ${\displaystyle y_{\bullet }={\left(y_j\right)}_{j\in J}}$ is a net in ${\displaystyle Y.}$ Make ${\displaystyle I\times J}$ into a directed set by declaring ${\displaystyle (i,j)\le \left(i_2,j_2\right)}$ if and only if ${\displaystyle i\le i_2\text{ and }j\le j_2.}$ Then ${\displaystyle x_{\bullet }\times y_{\bullet }:={\left(x_i,y_j\right)}_{(i,j)\in I\times J}}$ denotes the product net. If ${\displaystyle X=Y}$ then the image of this net under the addition map ${\displaystyle X\times X\to X}$ denotes the sum of these two nets:

$${\displaystyle x_{\bullet }+y_{\bullet }:={\left(x_i+y_j\right)}_{(i,j)\in I\times J}}$$

and similarly their difference is defined to be the image of the product net under the subtraction map:

$${\displaystyle x_{\bullet }-y_{\bullet }:={\left(x_i-y_j\right)}_{(i,j)\in I\times J}.}$$

A net ${\displaystyle x_{\bullet }={\left(x_i\right)}_{i\in I}}$ in an additive topological group ${\displaystyle X}$ is called a Cauchy net if

$${\displaystyle {\left(x_i-x_j\right)}_{(i,j)\in I\times I}\to 0\text{ in }X}$$

or equivalently, if for every neighborhood ${\displaystyle N}$ of ${\displaystyle 0}$ in ${\displaystyle X,}$ there exists some ${\displaystyle i_0\in I}$ such that ${\displaystyle x_i-x_j\in N}$ for all indices ${\displaystyle i,j\ge i_0.}$

A Cauchy sequence is a Cauchy net that is a sequence.

If ${\displaystyle B}$ is a subset of an additive group ${\displaystyle X}$ and ${\displaystyle N}$ is a set containing ${\displaystyle 0,}$ then${\displaystyle B}$ is said to be an ${\displaystyle N}$-small set or small of order ${\displaystyle N}$ if ${\displaystyle B-B\subseteq N.}$

A prefilter ${\displaystyle \mathcal{B}}$ on an additive topological group ${\displaystyle X}$ called a Cauchy prefilter if it satisfies any of the following equivalent conditions:

1. ${\displaystyle \mathcal{B}-\mathcal{B}\to 0}$ in ${\displaystyle X,}$ where ${\displaystyle \mathcal{B}-\mathcal{B}:=\{B-C:B,C\in \mathcal{B}\}}$ is a prefilter.
2. ${\displaystyle \{B-B:B\in \mathcal{B}\}\to 0}$ in ${\displaystyle X,}$ where ${\displaystyle \{B-B:B\in \mathcal{B}\}}$ is a prefilter equivalent to ${\displaystyle \mathcal{B}-\mathcal{B}.}$
3. For every neighborhood ${\displaystyle N}$ of ${\displaystyle 0}$ in ${\displaystyle X,}$ ${\displaystyle \mathcal{B}}$ contains some ${\displaystyle N}$-small set (that is, there exists some ${\displaystyle B\in \mathcal{B}}$ such that ${\displaystyle B-B\subseteq N}$).

and if ${\displaystyle X}$ is commutative then also:

4. For every neighborhood ${\displaystyle N}$ of ${\displaystyle 0}$ in ${\displaystyle X,}$ there exists some ${\displaystyle B\in \mathcal{B}}$ and some ${\displaystyle x\in X}$ such that ${\displaystyle B\subseteq x+N.}$

• It suffices to check any of the above condition for any given neighborhood basis of ${\displaystyle 0}$ in ${\displaystyle X.}$

Suppose ${\displaystyle \mathcal{B}}$ is a prefilter on a commutative topological group ${\displaystyle X}$ and ${\displaystyle x\in X.}$ Then ${\displaystyle \mathcal{B}\to x}$ in ${\displaystyle X}$ if and only if ${\displaystyle x\in \mathrm{cl}\mathcal{B}}$ and ${\displaystyle \mathcal{B}}$ is Cauchy.

Complete commutative topological group

Main article: Complete uniform space

Recall that for any ${\displaystyle S\subseteq X,}$ a prefilter ${\displaystyle \mathcal{C}}$ on ${\displaystyle S}$ is necessarily a subset of ${\displaystyle \mathrm{\wp }(S)}$; that is, ${\displaystyle \mathcal{C}\subseteq \mathrm{\wp }(S).}$

A subset ${\displaystyle S}$ of a topological group ${\displaystyle X}$ is called a complete subset if it satisfies any of the following equivalent conditions:

1. Every Cauchy prefilter ${\displaystyle \mathcal{C}\subseteq \mathrm{\wp }(S)}$ on ${\displaystyle S}$ converges to at least one point of ${\displaystyle S.}$
   • If ${\displaystyle X}$ is Hausdorff then every prefilter on ${\displaystyle S}$ will converge to at most one point of ${\displaystyle X.}$ But if ${\displaystyle X}$ is not Hausdorff then a prefilter may converge to multiple points in ${\displaystyle X.}$ The same is true for nets.
2. Every Cauchy net in ${\displaystyle S}$ converges to at least one point of ${\displaystyle S}$;
3. Every Cauchy filter ${\displaystyle \mathcal{C}}$ on ${\displaystyle S}$ converges to at least one point of ${\displaystyle S.}$
4. ${\displaystyle S}$ is a complete uniform space (under the point-set topology definition of "complete uniform space") when ${\displaystyle S}$ is endowed with the uniformity induced on it by the canonical uniformity of ${\displaystyle X}$;

A subset ${\displaystyle S}$ is called a sequentially complete subset if every Cauchy sequence in ${\displaystyle S}$ (or equivalently, every elementary Cauchy filter/prefilter on ${\displaystyle S}$) converges to at least one point of ${\displaystyle S.}$

• Importantly, convergence outside of ${\displaystyle S}$ is allowed: If ${\displaystyle X}$ is not Hausdorff and if every Cauchy prefilter on ${\displaystyle S}$ converges to some point of ${\displaystyle S,}$ then ${\displaystyle S}$ will be complete even if some or all Cauchy prefilters on ${\displaystyle S}$ also converge to points(s) in the complement ${\displaystyle X\setminus S.}$ In short, there is no requirement that these Cauchy prefilters on ${\displaystyle S}$ converge only to points in ${\displaystyle S.}$ The same can be said of the convergence of Cauchy nets in ${\displaystyle S.}$
  • As a consequence, if a commutative topological group ${\displaystyle X}$ is not Hausdorff, then every subset of the closure of ${\displaystyle \{0\},}$ say ${\displaystyle S\subseteq \mathrm{cl}\{0\},}$ is complete (since it is clearly compact and every compact set is necessarily complete). So in particular, if ${\displaystyle S\ne \varnothing }$ (for example, if ${\displaystyle S}$ a is singleton set such as ${\displaystyle S=\{0\}}$) then ${\displaystyle S}$ would be complete even though every Cauchy net in ${\displaystyle S}$ (and every Cauchy prefilter on ${\displaystyle S}$), converges to every point in ${\displaystyle \mathrm{cl}\{0\}}$ (include those points in ${\displaystyle \mathrm{cl}\{0\}}$ that are not in ${\displaystyle S}$).
  • This example also shows that complete subsets (indeed, even compact subsets) of a non-Hausdorff space may fail to be closed (for example, if ${\displaystyle \varnothing \ne S\subseteq \mathrm{cl}\{0\}}$ then ${\displaystyle S}$ is closed if and only if ${\displaystyle S=\mathrm{cl}\{0\}}$).

A commutative topological group ${\displaystyle X}$ is called a complete group if any of the following equivalent conditions hold:

1. ${\displaystyle X}$ is complete as a subset of itself.
2. Every Cauchy net in ${\displaystyle X}$ converges to at least one point of ${\displaystyle X.}$
3. There exists a neighborhood of ${\displaystyle 0}$ in ${\displaystyle X}$ that is also a complete subset of ${\displaystyle X.}$
   • This implies that every locally compact commutative topological group is complete.
4. When endowed with its canonical uniformity, ${\displaystyle X}$ becomes is a complete uniform space.
   • In the general theory of uniform spaces, a uniform space is called a complete uniform space if each Cauchy filter in ${\displaystyle X}$ converges in ${\displaystyle (X,\tau )}$ to some point of ${\displaystyle X.}$

A topological group is called sequentially complete if it is a sequentially complete subset of itself.

Neighborhood basis: Suppose ${\displaystyle C}$ is a completion of a commutative topological group ${\displaystyle X}$ with ${\displaystyle X\subseteq C}$ and that ${\displaystyle \mathcal{N}}$ is a neighborhood base of the origin in ${\displaystyle X.}$ Then the family of sets

$${\displaystyle \left\{{\mathrm{cl}}_{C}N:N\in \mathcal{N}\right\}}$$

is a neighborhood basis at the origin in ${\displaystyle C.}$

Uniform continuity

Let ${\displaystyle X}$ and ${\displaystyle Y}$ be topological groups, ${\displaystyle D\subseteq X,}$ and ${\displaystyle f:D\to Y}$ be a map. Then ${\displaystyle f:D\to Y}$ is uniformly continuous if for every neighborhood ${\displaystyle U}$ of the origin in ${\displaystyle X,}$ there exists a neighborhood ${\displaystyle V}$ of the origin in ${\displaystyle Y}$ such that for all ${\displaystyle x,y\in D,}$ if ${\displaystyle y-x\in U}$ then ${\displaystyle f(y)-f(x)\in V.}$

Generalizations

Various generalizations of topological groups can be obtained by weakening the continuity conditions:

• A semitopological group is a group G with a topology such that for each c ∈ G the two functions G → G defined by x ↦ xc and x ↦ cx are continuous.
• A quasitopological group is a semitopological group in which the function mapping elements to their inverses is also continuous.
• A paratopological group is a group with a topology such that the group operation is continuous.

Topology

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Topology

A three-dimensional model of a figure-eight knot. The figure-eight knot is a prime knot and has an Alexander–Briggs notation of 4₁.

In mathematics, topology (from the Greek words τόπος, 'place, location', and λόγος, 'study') is concerned with the properties of a geometric object that are preserved under continuous deformations, such as stretching, twisting, crumpling, and bending; that is, without closing holes, opening holes, tearing, gluing, or passing through itself.

A topological space is a set endowed with a structure, called a topology, which allows defining continuous deformation of subspaces, and, more generally, all kinds of continuity. Euclidean spaces, and, more generally, metric spaces are examples of a topological space, as any distance or metric defines a topology. The deformations that are considered in topology are homeomorphisms and homotopies. A property that is invariant under such deformations is a topological property. The following are basic examples of topological properties: the dimension, which allows distinguishing between a line and a surface; compactness, which allows distinguishing between a line and a circle; connectedness, which allows distinguishing a circle from two non-intersecting circles.

The ideas underlying topology go back to Gottfried Leibniz, who in the 17th century envisioned the geometria situs and analysis situs. Leonhard Euler's Seven Bridges of Königsberg problem and polyhedron formula are arguably the field's first theorems. The term topology was introduced by Johann Benedict Listing in the 19th century; although, it was not until the first decades of the 20th century that the idea of a topological space was developed.

Motivation

Möbius strips, which have only one surface and one edge, are a kind of object studied in topology.

The motivating insight behind topology is that some geometric problems depend not on the exact shape of the objects involved, but rather on the way they are put together. For example, the square and the circle have many properties in common: they are both one dimensional objects (from a topological point of view) and both separate the plane into two parts, the part inside and the part outside.

In one of the first papers in topology, Leonhard Euler demonstrated that it was impossible to find a route through the town of Königsberg (now Kaliningrad) that would cross each of its seven bridges exactly once. This result did not depend on the lengths of the bridges or on their distance from one another, but only on connectivity properties: which bridges connect to which islands or riverbanks. This Seven Bridges of Königsberg problem led to the branch of mathematics known as graph theory.

Similarly, the hairy ball theorem of algebraic topology says that "one cannot comb the hair flat on a hairy ball without creating a cowlick." This fact is immediately convincing to most people, even though they might not recognize the more formal statement of the theorem, that there is no nonvanishing continuous tangent vector field on the sphere. As with the Bridges of Königsberg, the result does not depend on the shape of the sphere; it applies to any kind of smooth blob, as long as it has no holes.

To deal with these problems that do not rely on the exact shape of the objects, one must be clear about just what properties these problems do rely on. From this need arises the notion of homeomorphism. The impossibility of crossing each bridge just once applies to any arrangement of bridges homeomorphic to those in Königsberg, and the hairy ball theorem applies to any space homeomorphic to a sphere.

Intuitively, two spaces are homeomorphic if one can be deformed into the other without cutting or gluing. A traditional joke is that a topologist cannot distinguish a coffee mug from a doughnut, since a sufficiently pliable doughnut could be reshaped to a coffee cup by creating a dimple and progressively enlarging it, while shrinking the hole into a handle.

Homeomorphism can be considered the most basic topological equivalence. Another is homotopy equivalence. This is harder to describe without getting technical, but the essential notion is that two objects are homotopy equivalent if they both result from "squishing" some larger object.

History

The Seven Bridges of Königsberg was a problem solved by Euler.

See also: History of the separation axioms

Topology, as a well-defined mathematical discipline, originates in the early part of the twentieth century, but some isolated results can be traced back several centuries. Among these are certain questions in geometry investigated by Leonhard Euler. His 1736 paper on the Seven Bridges of Königsberg is regarded as one of the first practical applications of topology. On 14 November 1750, Euler wrote to a friend that he had realized the importance of the edges of a polyhedron. This led to his polyhedron formula, V − E + F = 2 (where V, E, and F respectively indicate the number of vertices, edges, and faces of the polyhedron). Some authorities regard this analysis as the first theorem, signaling the birth of topology.

Further contributions were made by Augustin-Louis Cauchy, Ludwig Schläfli, Johann Benedict Listing, Bernhard Riemann and Enrico Betti. Listing introduced the term "Topologie" in Vorstudien zur Topologie, written in his native German, in 1847, having used the word for ten years in correspondence before its first appearance in print. The English form "topology" was used in 1883 in Listing's obituary in the journal Nature to distinguish "qualitative geometry from the ordinary geometry in which quantitative relations chiefly are treated".

Their work was corrected, consolidated and greatly extended by Henri Poincaré. In 1895, he published his ground-breaking paper on Analysis Situs, which introduced the concepts now known as homotopy and homology, which are now considered part of algebraic topology.

Topological characteristics of closed 2-manifolds

Manifold	Euler num	Orientability	Betti numbers			Torsion coefficient (1-dim)
			b0	b1	b2
Sphere	2	Orientable	1	0	1	none
Torus	0	Orientable	1	2	1	none
2-holed torus	−2	Orientable	1	4	1	none
g-holed torus (genus g)	2 − 2g	Orientable	1	2g	1	none
Projective plane	1	Non-orientable	1	0	0	2
Klein bottle	0	Non-orientable	1	1	0	2
Sphere with c cross-caps (c > 0)	2 − c	Non-orientable	1	c − 1	0	2
2-Manifold with g holes and c cross-caps (c > 0)	2 − (2g + c)	Non-orientable	1	(2g + c) − 1	0	2

Unifying the work on function spaces of Georg Cantor, Vito Volterra, Cesare Arzelà, Jacques Hadamard, Giulio Ascoli and others, Maurice Fréchet introduced the metric space in 1906. A metric space is now considered a special case of a general topological space, with any given topological space potentially giving rise to many distinct metric spaces. In 1914, Felix Hausdorff coined the term "topological space" and gave the definition for what is now called a Hausdorff space. Currently, a topological space is a slight generalization of Hausdorff spaces, given in 1922 by Kazimierz Kuratowski.

Modern topology depends strongly on the ideas of set theory, developed by Georg Cantor in the later part of the 19th century. In addition to establishing the basic ideas of set theory, Cantor considered point sets in Euclidean space as part of his study of Fourier series. For further developments, see point-set topology and algebraic topology.

The 2022 Abel Prize was awarded to Dennis Sullivan "for his groundbreaking contributions to topology in its broadest sense, and in particular its algebraic, geometric and dynamical aspects".

Concepts

Topologies on sets

Main article: Topological space

The term topology also refers to a specific mathematical idea central to the area of mathematics called topology. Informally, a topology describes how elements of a set relate spatially to each other. The same set can have different topologies. For instance, the real line, the complex plane, and the Cantor set can be thought of as the same set with different topologies.

Formally, let X be a set and let τ be a family of subsets of X. Then τ is called a topology on X if:

1. Both the empty set and X are elements of τ.
2. Any union of elements of τ is an element of τ.
3. Any intersection of finitely many elements of τ is an element of τ.

If τ is a topology on X, then the pair (X, τ) is called a topological space. The notation X_τ may be used to denote a set X endowed with the particular topology τ. By definition, every topology is a π-system.

The members of τ are called open sets in X. A subset of X is said to be closed if its complement is in τ (that is, its complement is open). A subset of X may be open, closed, both (a clopen set), or neither. The empty set and X itself are always both closed and open. An open subset of X which contains a point x is called a neighborhood of x.

Continuous functions and homeomorphisms

A continuous transformation can turn a coffee mug into a donut.
Ceramic model by Keenan Crane and Henry Segerman.

Main articles: Continuous function and homeomorphism

A function or map from one topological space to another is called continuous if the inverse image of any open set is open. If the function maps the real numbers to the real numbers (both spaces with the standard topology), then this definition of continuous is equivalent to the definition of continuous in calculus. If a continuous function is one-to-one and onto, and if the inverse of the function is also continuous, then the function is called a homeomorphism and the domain of the function is said to be homeomorphic to the range. Another way of saying this is that the function has a natural extension to the topology. If two spaces are homeomorphic, they have identical topological properties, and are considered topologically the same. The cube and the sphere are homeomorphic, as are the coffee cup and the doughnut. However, the sphere is not homeomorphic to the doughnut.

Manifolds

Main article: Manifold

While topological spaces can be extremely varied and exotic, many areas of topology focus on the more familiar class of spaces known as manifolds. A manifold is a topological space that resembles Euclidean space near each point. More precisely, each point of an n-dimensional manifold has a neighborhood that is homeomorphic to the Euclidean space of dimension n. Lines and circles, but not figure eights, are one-dimensional manifolds. Two-dimensional manifolds are also called surfaces, although not all surfaces are manifolds. Examples include the plane, the sphere, and the torus, which can all be realized without self-intersection in three dimensions, and the Klein bottle and real projective plane, which cannot (that is, all their realizations are surfaces that are not manifolds).

Topics

General topology

Main article: General topology

General topology is the branch of topology dealing with the basic set-theoretic definitions and constructions used in topology. It is the foundation of most other branches of topology, including differential topology, geometric topology, and algebraic topology. Another name for general topology is point-set topology.

The basic object of study is topological spaces, which are sets equipped with a topology, that is, a family of subsets, called open sets, which is closed under finite intersections and (finite or infinite) unions. The fundamental concepts of topology, such as continuity, compactness, and connectedness, can be defined in terms of open sets. Intuitively, continuous functions take nearby points to nearby points. Compact sets are those that can be covered by finitely many sets of arbitrarily small size. Connected sets are sets that cannot be divided into two pieces that are far apart. The words nearby, arbitrarily small, and far apart can all be made precise by using open sets. Several topologies can be defined on a given space. Changing a topology consists of changing the collection of open sets. This changes which functions are continuous and which subsets are compact or connected.

Metric spaces are an important class of topological spaces where the distance between any two points is defined by a function called a metric. In a metric space, an open set is a union of open disks, where an open disk of radius r centered at x is the set of all points whose distance to x is less than r. Many common spaces are topological spaces whose topology can be defined by a metric. This is the case of the real line, the complex plane, real and complex vector spaces and Euclidean spaces. Having a metric simplifies many proofs.

Algebraic topology

Main article: Algebraic topology

Algebraic topology is a branch of mathematics that uses tools from algebra to study topological spaces. The basic goal is to find algebraic invariants that classify topological spaces up to homeomorphism, though usually most classify up to homotopy equivalence.

The most important of these invariants are homotopy groups, homology, and cohomology.

Although algebraic topology primarily uses algebra to study topological problems, using topology to solve algebraic problems is sometimes also possible. Algebraic topology, for example, allows for a convenient proof that any subgroup of a free group is again a free group.

Differential topology

Main article: Differential topology

Differential topology is the field dealing with differentiable functions on differentiable manifolds. It is closely related to differential geometry and together they make up the geometric theory of differentiable manifolds.

More specifically, differential topology considers the properties and structures that require only a smooth structure on a manifold to be defined. Smooth manifolds are "softer" than manifolds with extra geometric structures, which can act as obstructions to certain types of equivalences and deformations that exist in differential topology. For instance, volume and Riemannian curvature are invariants that can distinguish different geometric structures on the same smooth manifold – that is, one can smoothly "flatten out" certain manifolds, but it might require distorting the space and affecting the curvature or volume.

Geometric topology

Main article: Geometric topology

Geometric topology is a branch of topology that primarily focuses on low-dimensional manifolds (that is, spaces of dimensions 2, 3, and 4) and their interaction with geometry, but it also includes some higher-dimensional topology. Some examples of topics in geometric topology are orientability, handle decompositions, local flatness, crumpling and the planar and higher-dimensional Schönflies theorem.

In high-dimensional topology, characteristic classes are a basic invariant, and surgery theory is a key theory.

Low-dimensional topology is strongly geometric, as reflected in the uniformization theorem in 2 dimensions – every surface admits a constant curvature metric; geometrically, it has one of 3 possible geometries: positive curvature/spherical, zero curvature/flat, and negative curvature/hyperbolic – and the geometrization conjecture (now theorem) in 3 dimensions – every 3-manifold can be cut into pieces, each of which has one of eight possible geometries.

2-dimensional topology can be studied as complex geometry in one variable (Riemann surfaces are complex curves) – by the uniformization theorem every conformal class of metrics is equivalent to a unique complex one, and 4-dimensional topology can be studied from the point of view of complex geometry in two variables (complex surfaces), though not every 4-manifold admits a complex structure.

Generalizations

Occasionally, one needs to use the tools of topology but a "set of points" is not available. In pointless topology one considers instead the lattice of open sets as the basic notion of the theory, while Grothendieck topologies are structures defined on arbitrary categories that allow the definition of sheaves on those categories, and with that the definition of general cohomology theories.

Applications

Biology

Topology has been used to study various biological systems including molecules and nanostructure (e.g., membraneous objects). In particular, circuit topology and knot theory have been extensively applied to classify and compare the topology of folded proteins and nucleic acids. Circuit topology classifies folded molecular chains based on the pairwise arrangement of their intra-chain contacts and chain crossings. Knot theory, a branch of topology, is used in biology to study the effects of certain enzymes on DNA. These enzymes cut, twist, and reconnect the DNA, causing knotting with observable effects such as slower electrophoresis. Topology is also used in evolutionary biology to represent the relationship between phenotype and genotype. Phenotypic forms that appear quite different can be separated by only a few mutations depending on how genetic changes map to phenotypic changes during development. In neuroscience, topological quantities like the Euler characteristic and Betti number have been used to measure the complexity of patterns of activity in neural networks.

Computer science

Topological data analysis uses techniques from algebraic topology to determine the large scale structure of a set (for instance, determining if a cloud of points is spherical or toroidal). The main method used by topological data analysis is to:

1. Replace a set of data points with a family of simplicial complexes, indexed by a proximity parameter.
2. Analyse these topological complexes via algebraic topology – specifically, via the theory of persistent homology.
3. Encode the persistent homology of a data set in the form of a parameterized version of a Betti number, which is called a barcode.

Several branches of programming language semantics, such as domain theory, are formalized using topology. In this context, Steve Vickers, building on work by Samson Abramsky and Michael B. Smyth, characterizes topological spaces as Boolean or Heyting algebras over open sets, which are characterized as semidecidable (equivalently, finitely observable) properties.

Physics

Topology is relevant to physics in areas such as condensed matter physics, quantum field theory and physical cosmology.

The topological dependence of mechanical properties in solids is of interest in disciplines of mechanical engineering and materials science. Electrical and mechanical properties depend on the arrangement and network structures of molecules and elementary units in materials. The compressive strength of crumpled topologies is studied in attempts to understand the high strength to weight of such structures that are mostly empty space. Topology is of further significance in Contact mechanics where the dependence of stiffness and friction on the dimensionality of surface structures is the subject of interest with applications in multi-body physics.

A topological quantum field theory (or topological field theory or TQFT) is a quantum field theory that computes topological invariants.

Although TQFTs were invented by physicists, they are also of mathematical interest, being related to, among other things, knot theory, the theory of four-manifolds in algebraic topology, and to the theory of moduli spaces in algebraic geometry. Donaldson, Jones, Witten, and Kontsevich have all won Fields Medals for work related to topological field theory.

The topological classification of Calabi–Yau manifolds has important implications in string theory, as different manifolds can sustain different kinds of strings.

In cosmology, topology can be used to describe the overall shape of the universe. This area of research is commonly known as spacetime topology.

In condensed matter a relevant application to topological physics comes from the possibility to obtain one-way current, which is a current protected from backscattering. It was first discovered in electronics with the famous quantum Hall effect, and then generalized in other areas of physics, for instance in photonics by F.D.M Haldane.

Robotics

The possible positions of a robot can be described by a manifold called configuration space. In the area of motion planning, one finds paths between two points in configuration space. These paths represent a motion of the robot's joints and other parts into the desired pose.

Games and puzzles

Disentanglement puzzles are based on topological aspects of the puzzle's shapes and components.

Fiber art

In order to create a continuous join of pieces in a modular construction, it is necessary to create an unbroken path in an order which surrounds each piece and traverses each edge only once. This process is an application of the Eulerian path.