In topology and related fields of mathematics, a sequential space is a topological space whose topology can be completely characterized by its convergent/divergent sequences. They can be thought of as spaces that satisfy a very weak axiom of countability, and all first-countable spaces (especially metric spaces) are sequential.

In any topological space ${\displaystyle (X,\tau ),}$ if a convergent sequence is contained in a closed set ${\displaystyle C,}$ then the limit of that sequence must be contained in ${\displaystyle C}$ as well. Sets with this property are known as sequentially closed. Sequential spaces are precisely those topological spaces for which sequentially closed sets are in fact closed. (These definitions can also be rephrased in terms of sequentially open sets; see below.) Said differently, any topology can be described in terms of nets (also known as Moore–Smith sequences), but those sequences may be "too long" (indexed by too large an ordinal) to compress into a sequence. Sequential spaces are those topological spaces for which nets of countable length (i.e., sequences) suffice to describe the topology.

Any topology can be refined (that is, made finer) to a sequential topology, called the sequential coreflection of ${\displaystyle X.}$

The related concepts of Fréchet–Urysohn spaces, T-sequential spaces, and ${\displaystyle N}$-sequential spaces are also defined in terms of how a space's topology interacts with sequences, but have subtly different properties.

Sequential spaces and ${\displaystyle N}$-sequential spaces were introduced by S. P. Franklin.^[1]

History

Although spaces satisfying such properties had implicitly been studied for several years, the first formal definition is due to S. P. Franklin in 1965. Franklin wanted to determine "the classes of topological spaces that can be specified completely by the knowledge of their convergent sequences", and began by investigating the first-countable spaces, for which it was already known that sequences sufficed. Franklin then arrived at the modern definition by abstracting the necessary properties of first-countable spaces.

Preliminary definitions

Let ${\displaystyle X}$ be a set and let ${\displaystyle x_{\bullet }=\left(x_{i}\right)_{i=1}^{\infty }}$ be a sequence in ${\displaystyle X}$; that is, a family of elements of ${\displaystyle X}$, indexed by the natural numbers. In this article, ${\displaystyle x_{\bullet }\subseteq S}$ means that each element in the sequence ${\displaystyle x_{\bullet }}$ is an element of ${\displaystyle S,}$ and, if ${\displaystyle f:X\to Y}$ is a map, then ${\displaystyle f\left(x_{\bullet }\right)=\left(f\left(x_{i}\right)\right)_{i=1}^{\infty }.}$ For any index ${\displaystyle i,}$ the tail of ${\displaystyle x_{\bullet }}$ starting at ${\displaystyle i}$ is the sequence

$${\displaystyle x_{\geq i}=(x_{i},x_{i+1},x_{i+2},\ldots ){\text{.}}}$$

A sequence ${\displaystyle x_{\bullet }}$ is eventually in ${\displaystyle S}$ if some tail of ${\displaystyle x_{\bullet }}$ satisfies ${\displaystyle x_{\geq i}\subseteq S.}$

Let ${\displaystyle \tau }$ be a topology on ${\displaystyle X}$ and ${\displaystyle x_{\bullet }}$ a sequence therein. The sequence ${\displaystyle x_{\bullet }}$ converges to a point ${\displaystyle x\in X,}$ written ${\displaystyle x_{\bullet }{\overset {\tau }{\to }}x}$ (when context allows, ${\displaystyle x_{\bullet }\to x}$), if, for every neighborhood ${\displaystyle U\in \tau }$ of ${\displaystyle x,}$ eventually ${\displaystyle x_{\bullet }}$ is in ${\displaystyle U.}$ ${\displaystyle x}$ is then called a limit point of ${\displaystyle x_{\bullet }.}$

A function ${\displaystyle f:X\to Y}$ between topological spaces is sequentially continuous if ${\displaystyle x_{\bullet }\to x}$ implies ${\displaystyle f(x_{\bullet })\to f(x).}$

Sequential closure/interior

Let ${\displaystyle (X,\tau )}$ be a topological space and let ${\displaystyle S\subseteq X}$ be a subset. The topological closure (resp. topological interior) of ${\displaystyle S}$ in ${\displaystyle (X,\tau )}$ is denoted by ${\displaystyle \operatorname {cl} _{X}S}$ (resp. ${\displaystyle \operatorname {int} _{X}S}$).

The sequential closure of ${\displaystyle S}$ in ${\displaystyle (X,\tau )}$ is the set

$${\displaystyle \operatorname {scl} (S)=\left\{x\in X:{\text{there exists a sequence }}s_{\bullet }\subseteq S{\text{ such that }}s_{\bullet }\to x\right\}}$$

which defines a map, the sequential closure operator, on the power set of ${\displaystyle X.}$ If necessary for clarity, this set may also be written ${\displaystyle \operatorname {scl} _{X}(S)}$ or ${\displaystyle \operatorname {scl} _{(X,\tau )}(S).}$ It is always the case that ${\displaystyle \operatorname {scl} _{X}S\subseteq \operatorname {cl} _{X}S,}$ but the reverse may fail. The sequential interior of ${\displaystyle S}$ in ${\displaystyle (X,\tau )}$ is the set

$${\displaystyle \operatorname {sint} (S)=\{s\in S:{\text{whenever }}x_{\bullet }\subseteq X{\text{ and }}x_{\bullet }\to s,{\text{ then }}x_{\bullet }{\text{ is eventually in }}S\}}$$

(the topological space again indicated with a subscript if necessary).

Sequential closure and interior satisfy many of the nice properties of topological closure and interior: for all subsets ${\displaystyle R,S\subseteq X,}$

• ${\displaystyle \operatorname {scl} _{X}(X\setminus S)=X\setminus \operatorname {sint} _{X}(S)}$ and ${\displaystyle \operatorname {sint} _{X}(X\setminus S)=X\setminus \operatorname {scl} _{X}(S)}$;

  Proof

  Fix ${\displaystyle x\in \operatorname {sint} (X\setminus S).}$ If ${\displaystyle x\in \operatorname {scl} (S),}$ then there exists ${\displaystyle s_{\bullet }\subseteq S}$ with ${\displaystyle s_{\bullet }\to x.}$ But by the definition of sequential interior, eventually ${\displaystyle s_{\bullet }}$ is in ${\displaystyle X\setminus S,}$ contradicting ${\displaystyle s_{\bullet }\subseteq S.}$ Conversely, suppose ${\displaystyle x\notin \operatorname {sint} (X\setminus S)}$; then there exists a sequence ${\displaystyle s_{\bullet }\subseteq X}$ with ${\displaystyle s_{\bullet }\to x}$ that is not eventually in ${\displaystyle X\setminus S.}$ By passing to the subsequence of elements not in ${\displaystyle X\setminus S,}$ we may assume that ${\displaystyle s_{\bullet }\subseteq S.}$ But then ${\displaystyle x\in \operatorname {scl} (S).}$ ▮

• ${\displaystyle \operatorname {scl} (\emptyset )=\emptyset }$ and ${\displaystyle \operatorname {sint} (\emptyset )=\emptyset }$;
• ${\textstyle \operatorname {sint} (S)\subseteq S\subseteq \operatorname {scl} (S)}$;
• ${\displaystyle \operatorname {scl} (R\cup S)=\operatorname {scl} (R)\cup \operatorname {scl} (S)}$; and
• ${\textstyle \operatorname {scl} (S)\subseteq \operatorname {scl} (\operatorname {scl} (S)).}$

That is, sequential closure is a preclosure operator. Unlike topological closure, sequential closure is not idempotent: the last containment may be strict. Thus sequential closure is not a (Kuratowski) closure operator.

Sequential spaces and coreflection

As discussed above, sequential closure is not in general idempotent, and so not the closure operator of a topology. One can obtain an idempotent sequential closure via transfinite iteration: for a successor ordinal ${\displaystyle \alpha +1,}$ define (as usual)

$${\displaystyle (\operatorname {scl} )^{\alpha +1}(S)=\operatorname {scl} ((\operatorname {scl} )^{\alpha }(S))}$$

and, for a limit ordinal ${\displaystyle \alpha ,}$ define

$${\displaystyle (\operatorname {scl} )^{\alpha }(S)=\bigcup _{\beta <\alpha }{(\operatorname {scl} )^{\beta }(S)}{\text{.}}}$$

This process gives an ordinal-indexed increasing sequence of sets; as it turns out, that sequence always stabilizes by index ${\displaystyle \omega _{1}}$ (the first uncountable ordinal). Conversely, the sequential order of ${\displaystyle X}$ is the minimal ordinal at which, for any choice of ${\displaystyle S,}$ the above sequence will stabilize.^[2]

The transfinite sequential closure of ${\displaystyle S}$ is the terminal set in the above sequence: ${\displaystyle (\operatorname {scl} )^{\omega _{1}}(S).}$ The operator ${\displaystyle (\operatorname {scl} )^{\omega _{1}}}$ is idempotent and thus a closure operator. In particular, it defines a topology, the sequential coreflection. In the sequential coreflection, every sequentially-closed set is closed (and every sequentially-open set is open).^[3]

T- and N-sequential spaces

A T-sequential space is a topological space with sequential order 1, which is equivalent to any of the following conditions:^[1]

• The sequential closure (or interior) of every subset of ${\displaystyle X}$ is sequentially closed (resp. open).
• ${\displaystyle \operatorname {scl} }$ or ${\displaystyle \operatorname {sint} }$ are idempotent.
• ${\textstyle \operatorname {scl} (S)=\bigcap _{{\text{sequentially closed }}C\supseteq S}{C}}$ or ${\textstyle \operatorname {sint} (S)=\bigcup _{{\text{sequentially open }}U\subseteq S}{U}}$
• Any sequential neighborhood of ${\displaystyle x\in X}$ can be shrunk to a sequentially-open set that contains ${\displaystyle x}$; formally, sequentially-open neighborhoods are a neighborhood basis for the sequential neighborhoods.
• For any ${\displaystyle x\in X}$ and any sequential neighborhood ${\displaystyle N}$ of ${\displaystyle x,}$ there exists a sequential neighborhood ${\displaystyle M}$ of ${\displaystyle x}$ such that, for every ${\displaystyle m\in M,}$ the set ${\displaystyle N}$ is a sequential neighborhood of ${\displaystyle m.}$

Being a T-sequential space is incomparable with being a sequential space; there are sequential spaces that are not T-sequential and vice-versa. However, a topological space ${\displaystyle (X,\tau )}$ is called a ${\displaystyle N}$-sequential (or neighborhood-sequential) if it is both sequential and T-sequential. An equivalent condition is that every sequential neighborhood contains an open (classical) neighborhood.^[1]

Every first-countable space (and thus every metrizable space) is ${\displaystyle N}$-sequential. There exist topological vector spaces that are sequential but not ${\displaystyle N}$-sequential (and thus not T-sequential).^[1]

Examples and sufficient conditions

Every CW-complex is sequential, as it can be considered as a quotient of a metric space.

The prime spectrum of a commutative Noetherian ring with the Zariski topology is sequential. ^[7]

Take the real line ${\displaystyle \mathbb {R} }$ and identify the set ${\displaystyle \mathbb {Z} }$ of integers to a point. As a quotient of a metric space, the result is sequential, but it is not first countable.

Every first-countable space is Fréchet–Urysohn and every Fréchet-Urysohn space is sequential. Thus every metrizable or pseudometrizable space — in particular, every second-countable space, metric space, or discrete space — is sequential.

Let ${\displaystyle {\mathcal {F}}}$ be a set of maps from Fréchet–Urysohn spaces to ${\displaystyle X.}$ Then the final topology that ${\displaystyle {\mathcal {F}}}$ induces on ${\displaystyle X}$ is sequential.

A Hausdorff topological vector space is sequential if and only if there exists no strictly finer topology with the same convergent sequences.^[8][9]

Consequences

Every sequential space has countable tightness and is compactly generated.

If ${\displaystyle f:X\to Y}$ is a continuous open surjection between two Hausdorff sequential spaces then the set ${\displaystyle \{y:{|f^{-1}(y)|=1}\}\subseteq Y}$ of points with unique preimage is closed. (By continuity, so is its preimage in ${\displaystyle X,}$ the set of all points on which ${\displaystyle f}$ is injective.)

If ${\displaystyle f:X\to Y}$ is a surjective map (not necessarily continuous) onto a Hausdorff sequential space ${\displaystyle Y}$ and ${\displaystyle {\mathcal {B}}}$ bases for the topology on ${\displaystyle X,}$ then ${\displaystyle f:X\to Y}$ is an open map if and only if, for every ${\displaystyle x\in X,}$ basic neighborhood ${\displaystyle B\in {\mathcal {B}}}$ of ${\displaystyle x,}$ and sequence ${\displaystyle y_{\bullet }=\left(y_{i}\right)_{i=1}^{\infty }\to f(x)}$ in ${\displaystyle Y,}$ there is a subsequence of ${\displaystyle y_{\bullet }}$ that is eventually in ${\displaystyle f(B).}$

Categorical properties

The full subcategory Seq of all sequential spaces is closed under the following operations in the category Top of topological spaces:

The category Seq is not closed under the following operations in Top:

Since they are closed under topological sums and quotients, the sequential spaces form a coreflective subcategory of the category of topological spaces. In fact, they are the coreflective hull of metrizable spaces (that is, the smallest class of topological spaces closed under sums and quotients and containing the metrizable spaces).

The subcategory Seq is a Cartesian closed category with respect to its own product (not that of Top). The exponential objects are equipped with the (convergent sequence)-open topology.

P.I. Booth and A. Tillotson have shown that Seq is the smallest Cartesian closed subcategory of Top containing the underlying topological spaces of all metric spaces, CW-complexes, and differentiable manifolds and that is closed under colimits, quotients, and other "certain reasonable identities" that Norman Steenrod described as "convenient".^[17].

Every sequential space is compactly generated, and finite products in Seq coincide with those for compactly generated spaces, since products in the category of compactly generated spaces preserve quotients of metric spaces.

See also

• Axiom of countability – property of certain mathematical objects (usually in a category) that asserts the existence of a countable set with certain properties. Without such an axiom, such a set might not probably exist.Pages displaying wikidata descriptions as a fallback
• Closed graph property – Graph of a map closed in the product space
• First-countable space – Topological space where each point has a countable neighbourhood basis
• Fréchet–Urysohn space – Topological space
• Sequence covering map

Notes

Citations

References

• Arkhangel'skii, A.V. and Pontryagin, L.S., General Topology I, Springer-Verlag, New York (1990) ISBN 3-540-18178-4.
• Arkhangel'skii, A V (1966). "Mappings and spaces" (PDF). Russian Mathematical Surveys. 21 (4): 115–162. Bibcode:1966RuMaS..21..115A. doi:10.1070/RM1966v021n04ABEH004169. ISSN 0036-0279. S2CID 250900871. Retrieved 10 February 2021.
• Akiz, Hürmet Fulya; Koçak, Lokman (2019). "Sequentially Hausdorff and full sequentially Hausdorff spaces". Communications Faculty of Science University of Ankara Series A1Mathematics and Statistics. 68 (2): 1724–1732. doi:10.31801/cfsuasmas.424418. ISSN 1303-5991. Retrieved 10 February 2021.
• Boone, James (1973). "A note on mesocompact and sequentially mesocompact spaces". Pacific Journal of Mathematics. 44 (1): 69–74. doi:10.2140/pjm.1973.44.69. ISSN 0030-8730.
• Booth, Peter; Tillotson, J. (1980). "Monoidal closed, Cartesian closed and convenient categories of topological spaces". Pacific Journal of Mathematics. 88 (1): 35–53. doi:10.2140/pjm.1980.88.35. ISSN 0030-8730. Retrieved 10 February 2021.
• Engelking, R., General Topology, Heldermann, Berlin (1989). Revised and completed edition.
• Foged, L. (1985). "A characterization of closed images of metric spaces". Proceedings of the American Mathematical Society. 95 (3): 487–490. doi:10.1090/S0002-9939-1985-0806093-3. ISSN 0002-9939.
• Franklin, S. (1965). "Spaces in which sequences suffice" (PDF). Fundamenta Mathematicae. 57 (1): 107–115. doi:10.4064/fm-57-1-107-115. ISSN 0016-2736.
• Franklin, S. (1967). "Spaces in which sequences suffice II" (PDF). Fundamenta Mathematicae. 61 (1): 51–56. doi:10.4064/fm-61-1-51-56. ISSN 0016-2736. Retrieved 10 February 2021.
• Goreham, Anthony, "Sequential Convergence in Topological Spaces", (2016)
• Gruenhage, Gary; Michael, Ernest; Tanaka, Yoshio (1984). "Spaces determined by point-countable covers". Pacific Journal of Mathematics. 113 (2): 303–332. doi:10.2140/pjm.1984.113.303. ISSN 0030-8730.
• Michael, E.A. (1972). "A quintuple quotient quest". General Topology and Its Applications. 2 (2): 91–138. doi:10.1016/0016-660X(72)90040-2. ISSN 0016-660X.
• Shou, Lin; Chuan, Liu; Mumin, Dai (1997). "Images on locally separable metric spaces". Acta Mathematica Sinica. 13 (1): 1–8. doi:10.1007/BF02560519. ISSN 1439-8516. S2CID 122383748.
• Steenrod, N. E. (1967). "A convenient category of topological spaces". The Michigan Mathematical Journal. 14 (2): 133–152. doi:10.1307/mmj/1028999711. Retrieved 10 February 2021.
• Trèves, François (2006) [1967]. Topological Vector Spaces, Distributions and Kernels. Mineola, N.Y.: Dover Publications. ISBN 978-0-486-45352-1. OCLC 853623322.
• Wilansky, Albert (2013). Modern Methods in Topological Vector Spaces. Mineola, New York: Dover Publications, Inc. ISBN 978-0-486-49353-4. OCLC 849801114.