topology (point-set topology, point-free topology)
see also differential topology, algebraic topology, functional analysis and topological homotopy theory
Basic concepts
fiber space, space attachment
Extra stuff, structure, properties
Kolmogorov space, Hausdorff space, regular space, normal space
sequentially compact, countably compact, locally compact, sigma-compact, paracompact, countably paracompact, strongly compact
Examples
Basic statements
closed subspaces of compact Hausdorff spaces are equivalently compact subspaces
open subspaces of compact Hausdorff spaces are locally compact
compact spaces equivalently have converging subnet of every net
continuous metric space valued function on compact metric space is uniformly continuous
paracompact Hausdorff spaces equivalently admit subordinate partitions of unity
injective proper maps to locally compact spaces are equivalently the closed embeddings
locally compact and second-countable spaces are sigma-compact
Theorems
Analysis Theorems
A topological space is called compactly generated – also called a “k-space”^{1} (Gale 1950, 1., following lectures by Hurewicz in 1948), “Kelley space” (Gabriel & Zisman 1967, III.4), or “kaonic space” (Postnikov 1982, p. 34) – if its topology is detected by the continuous images of compact Hausdorff spaces inside it.
As opposed to general topological spaces, compactly generated spaces form a cartesian closed category while still being general enough for most purposes of general topology, hence form a convenient category of topological spaces (Steenrod 1967) and as such have come to be commonly used in the foundations of algebraic topology and homotopy theory, especially in their modern guise as compactly generated weakly Hausdorff spaces, due to McCord 1969, Sec. 2.
($k$-continuous functions)
A function $f\colon X \to Y$ between underlying sets of topological spaces is called $k$-continuous if for all compact Hausdorff spaces $C$ and continuous functions $t \colon C \to X$ the composite $f \circ t \colon C \to Y$ is continuous.
(equivalent characterizations of compact generation)
The following conditions on a topological space $X$ are equivalent:
For all spaces $Y$ and all functions $f \colon X \to Y$ (of underlying sets), $f$ is continuous if and only if $f$ is $k$-continuous (Def. ).
There is a set $S$ (instead of a proper class) of compact Hausdorff spaces such that the previous condition holds for all $C \in S$.
$X$ is an identification space of a disjoint union of compact Hausdorff spaces.
A subspace $U \subseteq X$ is open if and only if the preimage $t^{-1}(U)$ under any continuous function $t \colon C \to X$ out of a compact Hausdorff space $C$ is open.
(k-spaces)
A topological space $X$ is a $k$-space if any (hence all) of the conditions in Prop. hold.
(terminology)
Some authors say that a $k$-space (Def. ) is compactly generated, while others reserve that term for a $k$-space which is also weak Hausdorff, meaning that the image of any $t\colon C\to X$ is closed (when $C$ is compact Hausdorff). Some authors (especially the early authors on the subject) go on to require a Hausdorff space, but this seems to be unnecessary.
Examples of compactly generated spaces include
Every compact Hausdorff space is compactly generated.
Note: it is not generally true that compact spaces are compactly generated, even if they are weakly Hausdorff. An example is the square of the one-point compactification of the rationals $\mathbb{Q}$ with its standard topology. See for example this MathStackExchange post.
Every locally compact Hausdorff space is compactly generated.
Every topological manifold is compactly generated
Every CW-complex is a compactly generated topological space.
Since a CW-complex $X$ is a colimit in Top over attachments of standard n-disks $D^{n_i}$ (its cells), by the characterization of colimits in $Top$ (prop.) a subset of $X$ is open or closed precisely if its restriction to each cell is open or closed, respectively. Since the $n$-disks are compact, this implies one direction: if a subset $A$ of $X$ intersected with all compact subsets is closed, then $A$ is closed.
For the converse direction, since a CW-complex is a Hausdorff space and since compact subspaces of Hausdorff spaces are closed, the intersection of a closed subset with a compact subset is closed.
Every first countable space is a compactly generated space.
Since the topology is determined by convergent sequences = maps from one-point compactification $\mathbb{N} \cup \{\infty\}$); these include all Frechet–Uryson spaces.
(category of k-spaces)
We write
for the category of $k$-spaces (Def. ) with continuous functions between them, hence for the full subcategory of Top on the k-spaces.
The inclusion (1) is that of a coreflective subcategory
The reflection functor $k$ is constructed as follows:
We take $k(X) \coloneqq X$ on underlying sets, and equip this with the topology whose closed sets are those whose intersection with compact Hausdorff subsets of (the original topology on) $X$ is closed (in the original topology on $X$). Then $k(X)$ has all the same closed sets and possibly more, hence all the same open sets and possibly more.
In particular, the identity map $id \colon k(X)\to X$ is continuous, and forms the counit of the coreflection. Thus this coreflection has a counit which is both a monomorphism as well as an epimorphism, i.e. a “bimorphism”—such a coreflection is sometimes called a “bicoreflection.”
There is also the category $\Top_k$ of all topological spaces and $k$-continuous maps (Def. ). But the composite sequence of inclusions
of which the first is the full inclusion (1) and the second is bijective on objects $k\Top \to Top_k$, is an equivalence of categories.
Namely, the identity morphism $id \colon X \to k(X)$ is $k$-continuous, so that the adjunction counit from Prop. becomes an isomorphism in $\Top_k$. This shows that $k\Top \to \Top_k$ is essentially surjective, and it is fully faithful since any $k$-continuous function between $k$-spaces is $k$-continuous; hence it is an equivalence.
Since $k\Top \hookrightarrow \Top$ is coreflective, it follows that $k\Top$ is complete and cocomplete. Its colimits are constructed as in Top, but its limits are the $k$-ification (2) of limits in Top.
Notice that this is nontrivial already for products: the $k$-space product $X \times Y$ is the $k$-ification of the usual product topology. The $k$-space product is better behaved in many ways; e.g. it enables geometric realization to preserve products (and all finite limits), and the product of two CW complexes to be another CW complex.
Write
for the further full subcategory inside that of k-spaces (Def. ) on those which in addition are weak Hausdorff spaces.
(cgwh spaces reflective in cg spaces)
The full subcategory-inclusion of weak Hausdorff spaces in k-spaces (Def. ) is a reflective subcategory inclusion:
(sequence of (co-)reflections)
In summary, Prop. and Prop. yield a sequence of adjoint functors of this form:
The classical model structure on topological spaces restricts along these (co-)reflective embeddings to a Quillen equivalent model structure on compactly generated topological spaces. See there for more.
The categories $k\Top\simeq \Top_k$ are cartesian closed. (While in Top only some objects are exponentiable, see exponential law for spaces.) For arbitrary spaces $X$ and $Y$, define the test-open or compact-open topology on $\Top_k(X,Y)$ to have the subbase of sets $M(t,U)$, for a given compact Hausdorff space $C$, a map $t\colon C \to X$, and an open set $U$ in $Y$, where $M(t,U)$ consists of all $k$-continuous functions $f\colon X \to Y$ such that $f(t(C))\subseteq U$.
(This is slightly different from the usual compact-open topology if $X$ happens to have non-Hausdorff compact subspaces; in that case the usual definition includes such subspaces as tests, while the above definition excludes them. Of course, if $X$ itself is Hausdorff, then the two become identical.)
With this topology, $\Top_k(X,Y)$ becomes an exponential object in $Top_k$. It follows, by Yoneda lemma arguments (prop.), that the bijection
is actually an isomorphism in $\Top_k$, which we may call a $k$-homeomorphism (e.g. Strickland 09, prop. 2.12). In fact, it is actually a homeomorphism, i.e. an isomorphism already in $Top$.
It follows that the category $k\Top$ of $k$-spaces and continuous maps is also cartesian closed, since it is equivalent to $\Top_k$. Its exponential object is the $k$-ification of the one constructed above for $\Top_k$. Since for $k$-spaces, $k$-continuous implies continuous, the underlying set of this exponential space $k\Top(X,Y)$ is the set of all continuous maps from $X$ to $Y$. Thus, when $X$ is Hausdorff, we can identify this space with the $k$-ification of the usual compact-open topology on $Top(X,Y)$.
Finally, this all remains true if we also impose the weak Hausdorff, or Hausdorff, conditions.
(failure of local cartesian closure)
Unfortunately neither of the above categories is locally cartesian closed (Cagliari-Matovani-Vitale 1995, p. 4)
However, if $K$ is the category of not-necessarily-weak-Hausdorff k-spaces, and $A$ and $B$ are k-spaces that are weak Hausdorff, then the pullback (base change) functor $K_{/B} \to K_{/A}$ has a right adjoint. (see Booth & Brown 1978a, Thm. 3.5 & 7.3; May & Sigurdsson 2006, §1.3.7-§1.3.9).
There is still a lot of work on fibred exponential laws and their applications. One reason for the success and difficulties is that it is easy to give a topology on the space of closed subsets of a space $X$ by regarding this as the space of maps to the Sierpinski space (the set $\{0,1\}$ of truth values in which $\{1\}$ is closed but not open). From this one can get an exponential law for spaces over $B$ if $B$ is $T_0$, so that all fibres of spaces over $B$ are closed in their total space. Note that weak Hausdorff implies $T_0$.
Every locally compact Hausdorff space is a k-space and is weakly Hausdorff:
The product topological space of a locally compact Hausdorff space with a k-space is already a k-space (i.e. without need of k-ification).
(e.g. Lewis 1978, Lem. 2.4; Piccinini 1992, Thm. B.6, Strickland 2009, Prop. 2.6)
(k-spaces are the quotient spaces of locally compact Hausdorff spaces)
A topological space is a k-space (Def. ) iff it is a quotient topological space of a locally compact Hausdorff space.
(k-spaces are the colimits in Top of compact Hausdorff spaces)
A topological space is a k-space (Def. ) iff it is a colimit as formed in Top (according to this Prop.) of a diagram of compact Hausdorff spaces.
Insice $k$-spaces there is the further coreflective subcategory $D Top$ of Delta-generated topological spaces:
Both of these coreflections are Quillen equivalences with respect to the classical model structure on topological spaces and the induced model structure on compactly generated topological spaces and the model structure on Delta-generated topological spaces (Gaucher 2009, Haraguchi 2013).
(Cagliari-Matovani-Vitale 95, p. 3).
In particular this implies that in these categories pullback preserves effective epimorphisms (see there).
For every topological space $X$, the canonical continuous function from the $k$-ification (the adjunction counit) is a weak homotopy equivalence, hence induces an isomorphism on all homotopy groups:
The idea of compactly generated Hausdorff spaces first appears in print in:
where it is attributed to Witold Hurewicz, who introduced the concept in a lecture series given in Princeton, 1948-49, which Gale attended.^{2}
Early textbook accounts, assuming the Hausdorff condition:
John Kelley, p. 230 in: General topology, D. van Nostrand, New York (1955), reprinted as: Graduate Texts in Mathematics, Springer (1975) [ISBN:978-0-387-90125-1]
James Dugundji, Section XI.9 of: Topology, Allyn and Bacon 1966 (pdf)
Pierre Gabriel, Michel Zisman, sections I.1.5.3 and III.2 of Calculus of fractions and homotopy theory, Ergebnisse der Mathematik und ihrer Grenzgebiete, Band 35, Springer (1967) (pdf)
also:
Susan Niefield, Section 9 of: Cartesianness, PhD thesis, Rutgers 1978 (proquest:302920643)
Francis Borceux, Section 7.2 of: Categories and Structures, Vol. 2 of: Handbook of Categorical Algebra, Encyclopedia of Mathematics and its Applications 50 Cambridge University Press (1994) (doi:10.1017/CBO9780511525865)
Influential emphasis of the usefulness of the notion as providing a convenient category of topological spaces:
Early discussion in the context of geometric realization of simplicial topological spaces:
and briefly in:
More history and early references, with emphasis on category-theoretic aspects:
The terminology “kaonic spaces”, or rather the Russian version “каонные пространства” is used in
M M Postnikov, Введение в теорию Морса, Наука 1971 (web)
M M Postnikov, p. 34 of: Лекции по алгебраической топологии. Основы теории гомотопий, Наука 1982 (web)
Discussion of k-spaces in the generality of subcategory-generated spaces, including Delta-generated topological spaces:
Rainer M. Vogt, Convenient categories of topological spaces for homotopy theory, Arch. Math 22, 545–555 (1971) (doi:10.1007/BF01222616)
Martín Escardó, Jimmie Lawson, Alex Simpson, Section 3 of: Comparing Cartesian closed categories of (core) compactly generated spaces, Topology and its Applications Volume 143, Issues 1–3, 28 August 2004, Pages 105-145 (doi:10.1016/j.topol.2004.02.011)
Philippe Gaucher, Section 2 of: Homotopical interpretation of globular complex by multipointed d-space, Theory and Applications of Categories, vol. 22, number 22, 588-621, 2009 (arXiv:0710.3553)
Proof that k-spaces form a regular category:
Further accounts:
George W. Whitehead, Section I.4 of: Elements of Homotopy Theory, Springer 1978 (doi:10.1007/978-1-4612-6318-0)
Brian J. Day, Relationship of Spanier’s Quasi-topological Spaces to k-Spaces , M. Sc. thesis University of Sydney 1968. (pdf)
Peter Booth, Philip R. Heath, Renzo A. Piccinini, Fibre preserving maps and functional spaces, Algebraic topology (Proc. Conf., Univ. British Columbia, Vancouver, B.C., 1977), pp. 158–167, Lecture Notes in Math., 673, Springer, Berlin, 1978.
Renzo A. Piccinini, Appendix B in: Lectures on Homotopy Theory, Mathematics Studies 171, North Holland 1992 (ISBN:978-0-444-89238-6)
Yves Félix, Stephen Halperin, Jean-Claude Thomas, Section 0 of: Rational Homotopy Theory, Graduate Texts in Mathematics, 205, Springer-Verlag, 2000 (doi:10.1007/978-1-4613-0105-9)
(in a context of rational homotopy theory)
Marcelo Aguilar, Samuel Gitler, Carlos Prieto, around note 4.3.22 of Algebraic topology from a homotopical viewpoint, Springer (2002) (toc pdf)
Samuel Smith, The homotopy theory of function spaces: a survey (arXiv:1009.0804)
The idea of generalizing compact generation to weakly Hausdorff spaces appears in:
where it is attributed to John C. Moore.
Review in this generality of CG weakly Hausdorff spaces:
Gaunce Lewis, Compactly generated spaces (pdf), appendix A of The Stable Category and Generalized Thom Spectra, PhD thesis Chicago, 1978
Rudolf Fritsch, Renzo Piccinini, Appendix A.1 of: Cellular structures in topology, Cambridge University Press (1990) (doi:10.1017/CBO9780511983948, pdf)
Peter May, Chapter 5 of: A concise course in algebraic topology, University of Chicago Press 1999 (ISBN: 9780226511832, pdf)
Neil Strickland, The category of CGWH spaces, 2009 (pdf, pdf)
Stefan Schwede, Section A.2 of: Symmetric spectra (2012)
Charles Rezk, Compactly Generated Spaces, 2018 (pdf, pdf)
Brief review in preparation of the model structure on compactly generated topological spaces:
Review with focus on compactly generated topological G-spaces in equivariant homotopy theory and specifically equivariant bundle-theory:
On exponential objects (internal homs) in slice categories of (compactly generated) topological spaces – see at parameterized homotopy theory):
Peter I. Booth, The Exponential Law of Maps I, Proceedings of the London Mathematical Society s3-20 1 (1970) 179-192 $[$doi:10.1112/plms/s3-20.1.179$]$
Peter I. Booth, The exponential law of maps. II, Mathematische Zeitschrift 121 (1971) 311–319 $[$doi:10.1007/BF01109977$]$
Peter I. Booth, Ronnie Brown, Spaces of partial maps, fibred mapping spaces and the compact-open topology, General Topology and its Applications 8 2 (1978) 181-195 $[$doi:10.1016/0016-660X(78)90049-1$]$
Peter I. Booth, Ronnie Brown, On the application of fibred mapping spaces to exponential laws for bundles, ex-spaces and other categories of maps, General Topology and its Applications 8 2 (1978) 165-179 $[$doi:10.1016/0016-660X(78)90048-X$]$
L. Gaunce Lewis, Jr., §1 of: Open maps, colimits, and a convenient category of fibre spaces, Topology and its Applications 19 1 (1985) 75-89 $[$doi.org/10.1016/0166-8641(85)90087-2$]$
And with an eye towards parameterized homotopy theory:
Ioan Mackenzie James: §II.9 in: Fibrewise topology, Cambridge Tracts in Mathematics, Cambridge University Press (1989) $[$ISBN:9780521360906$]$
Peter May, Johann Sigurdsson, §1.3.7-§1.3.9 in: Parametrized Homotopy Theory, Mathematical Surveys and Monographs, vol. 132, AMS 2006 (ISBN:978-0-8218-3922-5, arXiv:math/0411656, pdf)
The reason for choosing the term “k-space” in Gale 1950 seems to be lost in history. The “k” is not for “Kelley”, as Kelley 1955 came later. It might have been an allusion to the German word kompakt. ↩
This is according to personal communication by David Gale to William Lawvere in 2003, forwarded by Lawvere to Martin Escardo at that time, and then kindly forwarded by Escardo to the nForum in 2021; see there. ↩
Last revised on March 28, 2024 at 21:15:11. See the history of this page for a list of all contributions to it.