In the field of topology, a Fréchet–Urysohn space is a topological space X with the property that for every subset the closure of S in X is identical to the sequential closure of S in X. Fréchet–Urysohn spaces are a special type of sequential space. The property is named after Maurice Fréchet and Pavel Urysohn.

Definitions

Let (X, \tau) be a topological space. The **** of**** S in**** (X, *tau**)*** is**** the set:**** where or may be written if clarity is needed. A topological space (X, *tau**)*** is**** said**** to**** be**** a **** if**** for every subset where denotes the closure of S in (X, \tau).

Sequentially open/closed sets

Suppose that**** is**** any subset**** of**** X.**** A sequence**** **** is**** **** if**** there exists**** a positive**** integer N such**** that**** x_i *in*** S for all indices i *geq** N.*** The set S is**** called**** **** if**** every sequence**** in**** X that**** converges to**** a point of**** S is**** eventually**** in**** S;**** Typically,**** if**** X is**** understood**** then**** **** is**** written in**** place of**** The set S is**** called**** **** if**** or**** equivalently****,**** if**** whenever**** **** is**** a sequence**** in**** S converging**** to**** x,**** then**** x must**** also**** be**** in**** S.**** The complement**** of**** a sequentially**** open**** set is**** a sequentially**** closed**** set,**** and vice**** versa.**** Let denote the set of all sequentially open subsets of (X, \tau), where this may be denoted by is the topology \tau is understood. The set is a topology on X that is finer than the original topology \tau. Every open (resp. closed) subset of X is sequentially open (resp. sequentially closed), which implies that

Strong Fréchet–Urysohn space

A topological space X is**** a **** if**** for every point x *in*** X and every sequence**** of**** subsets of**** the space X such**** that**** **** there exist a sequence**** **** in**** X such**** that**** ai *in*** Ai for every **** and **** in**** (X, *tau**)*.** The above properties can be expressed as selection principles.

Contrast to sequential spaces

Every open subset of X is sequentially open and every closed set is sequentially closed. However, the converses are in general not true. The spaces**** for which the converses are true**** are called**** ; that is, a sequential space is a topological space in which every sequentially open subset is necessarily open, or equivalently, it is a space in which every sequentially closed subset is necessarily closed. Every Fréchet-Urysohn space is a sequential space but there are sequential spaces that are not Fréchet-Urysohn spaces. Sequential spaces (respectively, Fréchet-Urysohn spaces) can be viewed/interpreted as exactly those spaces X where for any single given subset knowledge of which sequences in X converge to which point(s) of X (and which do not) is sufficient to **' S is closed in X (respectively, is sufficient to **' of S in X). Thus sequential spaces are those spaces X for which sequences in X can be used as a "test" to determine whether or not any given subset is open (or equivalently, closed) in X; or said differently, sequential spaces are those spaces whose topologies can be completely characterized in terms of sequence convergence. In any space that is sequential, there exists a subset for which this "test" gives a "false positive."

Characterizations

If (X, \tau) is a topological space then the following are equivalent:

<ol> <li>X is a Fréchet–Urysohn space.</li> <li>Definition: for every subset </li> <li> for every subset <li>Every subspace of X is a [sequential space](https://bliptext.com/articles/sequential-space).</li> ****<****l****i****>****F****o****r**** ****a****n****y**** ****s****u****b****s****e****t**** ****t****h****a****t**** ****i****s**** **** ****c****l****o****s****e****d**** ****i****n**** ****X**** ****a****n****d**** **** **** ****t****h****e****r****e**** ****e****x****i****s****t****s**** ****a**** ****s****e****q****u****e****n****c****e**** ****i****n**** ****S**** ****t****h****a****t**** ****c****o****n****v****e****r****g****e****s**** ****t****o**** ****x****.**** ****<****/****l****i****>**** </ol> The characterization below shows that from among Hausdorff sequential spaces, Fréchet–Urysohn spaces are exactly those for which a "[cofinal](https://bliptext.com/articles/cofinal-set) convergent diagonal sequence" can always be found, similar to the [diagonal principal](https://bliptext.com/articles/axiomatic-foundations-of-topological-spaces) that is used to [characterize topologies in terms of convergent nets](https://bliptext.com/articles/axiomatic-foundations-of-topological-spaces). In the following characterization, all convergence is assumed to take place in (X, \tau). If (X, \tau) is a [Hausdorff](https://bliptext.com/articles/hausdorff-space) [sequential space](https://bliptext.com/articles/sequential-space) then X is a Fréchet–Urysohn space if and only if the following condition holds: If is a sequence in X that converge to some x \in X and if for every l \in \N, is a sequence in X that converges to x_l, where these hypotheses can be summarized by the following diagram then there exist strictly increasing maps such that (It suffices to consider only sequences with infinite ranges (i.e. is infinite) because if it is finite then Hausdorffness implies that it is necessarily eventually constant with value x, in which case the existence of the maps with the desired properties is readily verified for this special case (even if (X, \tau) is not a Fréchet–Urysohn space).

Properties

Every subspace of a Fréchet–Urysohn space is Fréchet–Urysohn. Every Fréchet–Urysohn space is a sequential space although the opposite implication is not true in general. If a Hausdorff locally convex topological vector space (X, \tau) is a Fréchet-Urysohn space then \tau is equal to the final topology on X induced by the set of all arcs in (X, \tau), which by definition are continuous paths that are also topological embeddings.

Examples

Every first-countable space is a Fréchet–Urysohn space. Consequently, every second-countable space, every metrizable space, and every pseudometrizable space is a Fréchet–Urysohn space. It also follows that every topological space (X, \tau) on a finite set X is a Fréchet–Urysohn space.

Metrizable continuous dual spaces

A metrizable locally convex topological vector space (TVS) X (for example, a Fréchet space) is a normable space if and only if its strong dual space is a Fréchet–Urysohn space, or equivalently, if and only if is a normable space.

Sequential spaces that are not Fréchet–Urysohn

Direct limit of finite-dimensional Euclidean spaces **** *R*^{*i**nf**ty**}*** is**** a Hausdorff sequential**** space that**** is**** not Fréchet–Urysohn.**** For every integer n \geq 1, identify \R^n with the set where the latter is a subset of the space of sequences of real numbers explicitly, the elements and are identified together. In particular, \R^n can be identified as a subset of \R^{n+1} and more generally, as a subset for any integer k \geq 0. Let Give \R^{\infty} its usual topology \tau, in which a subset is open (resp. closed) if and only if for every integer n \geq 1, the set is an open (resp. closed) subset of \R^n (with it usual Euclidean topology). If and v_{\bull} is a sequence in \R^{\infty} then in if and only if there exists some integer n \geq 1 such that both v and v_{\bull} are contained in \R^n and in \R^n. From these facts, it follows that is a sequential space. For every integer n \geq 1, let B_n denote the open ball in \R^n of radius 1/n (in the Euclidean norm) centered at the origin. Let Then the closure of S is is all of \R^{\infty} but the origin of \R^{\infty} does belong to the sequential closure of S in In fact, it can be shown that This proves that is not a Fréchet–Urysohn space. Montel DF-spaces Every infinite-dimensional Montel DF-space is a sequential space but a Fréchet–Urysohn space. **The Schwartz space and the space of smooth functions ** The following extensively used spaces are prominent examples of sequential spaces that are not Fréchet–Urysohn spaces. Let denote the Schwartz space and let denote the space of smooth functions on an open subset where both of these spaces have their usual Fréchet space topologies, as defined in the article about distributions. Both and as well as the strong dual spaces of both these of spaces, are complete nuclear Montel ultrabornological spaces, which implies that all four of these locally convex spaces are also paracompact normal reflexive barrelled spaces. The strong dual spaces of both and are sequential spaces but of these duals is a Fréchet-Urysohn space.

Citations