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 pointed topological space is a topological space with a choice of one of its points.
Simplistic as this concept may seem, pointed topological spaces play a central role for instance in algebraic topology as domains for reduced generalized (Eilenberg-Steenrod) cohomology theories and as an ingredient for the definition of spectra.
One reason why pointed topological spaces are important is that the category which they form is an intermediate stage in the stabilization of homotopy theory (the classical homotopy theory of topological spaces) to stable homotopy theory:
The category of pointed topological spaces has a zero object (the point space itself) and the canonical tensor product on pointed spaces is the smash product, which is non-cartesian monoidal category, in contrast to the plain product of topological space.
A pointed topological space is a topological space $(X,\tau)$ equipped with a choice of point $x \in X$. A homomorphism between pointed topological space $(X,x)$ $(Y,y)$ is a continuous function $f \colon X \to Y$ which preserves the chosen basepoits in that $f(x) = y$.
Stated in the language of category theory, this mean that pointed topological spaces are the pointed objects in the category Top of topological spaces. This is the coslice category $Top^{\ast/}$ of topological spaces “under” the point space $\ast$:
an object in $Top^{\ast/}$ is equivalently a continuous function $x \colon \ast \to (X,\tau)$, which is equivalently just a choice of point in $X$, and a morphism in $Top^{\ast/}$ is a morphism $f \colon X \to Y$ in Top (hence a continuous function), such that this triangle diagram commutes
which equivalently means that $f(x) = y$.
The forgetful functor $Top^{\ast/} \to Top$ has a left adjoint given by forming the disjoint union space (coproduct in Top) with a point space (“adjoining a base point”), this is denoted by
Given two pointed topoligical spaces $(X,x)$ and $(Y,y)$, then:
their Cartesian product in $Top^{\ast/}$ is simply their product topological space $X \times Y$ equipped with the pair of basepoints $(X\times Y, (x,y))$;
their coproduct in $Top^{\ast/}$ has to be computed using the second clause in this prop.: since the point $\ast$ has to be adjoined to the diagram, it is given not by the coproduct in $Top$ (which is the disjoint union space), but by the pushout in $Top$ of the form:
This is called the wedge sum operation on pointed objects.
This is the quotient topological space of the disjoint union space under the equivalence relation which identifies the two basepoints:
Generally for a set $\{(X_i,x_i)\}_{i \in I}$ of pointed topological spaces
their product is formed in Top, as the product topological space with the Tychonoff topology, with the tuple $(x_i)_{i \in I} \in \underset{i \in I}{\prod} X_i$ of basepoints being the new basepoint;
their coproduct is formed by the colimit in $Top$ over the diagram with a basepoint adjoined, and is called the wedge sum $\vee_{i \in I} X_i$, which is the quotient topological space of the disjoint union space with all the basepoints identified:
For $X$ a CW-complex, then for every $n \in \mathbb{N}$ the quotient of its $n$-skeleton by its $(n-1)$-skeleton is the wedge sum, def. 1, of $n$-spheres, one for each $n$-cell of $X$:
The smash product of pointed topological spaces is the functor
given by
hence by the pushout in $Top$ of he frm
In terms of the wedge sum from def. 1, this may be written concisely as the quotient space (this def) of the product topological space by the subspace constituted by the wedge sum
t $\,$
symbol | name | category theory |
---|---|---|
$X \times Y$ | product space | product in $Top^{\ast/}$ |
$X \vee Y$ | wedge sum | coproduct in $Top^{\ast/}$ |
$X \wedge Y = \frac{X \times Y}{X \vee Y}$ | smash product | tensor product in $Top^{\ast/}$ |
For $X, Y \in Top$, with $X_+,Y_+ \in Top^{\ast/}$, def. 1, then
$X_+ \vee Y_+ \simeq (X \sqcup Y)_+$;
$X_+ \wedge Y_+ \simeq (X \times Y)_+$.
By example 1, $X_+ \vee Y_+$ is given by the colimit in $Top$ over the diagram
This is clearly $A \sqcup \ast \sqcup B$. Then, by definition 2
Let $I \coloneqq [0,1] \subset \mathbb{R}$ be the closed interval with its Euclidean metric topology.
Hence
is the interval with a disjoint basepoint adjoined, def. 1.
Now for $X$ any pointed topological space, then the smash product (def. 2)
is the reduced cylinder over $X$: the result of forming the ordinary cylinder over $X$, and then identifying the interval over the basepoint of $X$ with the point.
(Generally, any construction in $Top$ properly adapted to pointed spaces is called the “reduced” version of the unpointed construction. Notably so for “reduced suspension” which we come to below.)
Just like the ordinary cylinder $X\times I$ receives a canonical injection from the coproduct $X \sqcup X$ formed in $Top$, so the reduced cyclinder receives a canonical injection from the coproduct $X \sqcup X$ formed in $Top^{\ast/}$, which is the wedge sum from example 1:
Recall that the cone on a topological space $X$ is the quotient space of the product space with the closed interval
If $X$ is pointed with basepoint $x \in X$, then the reduced cone is the further quotient by the copy of the interval over the basepoint
For $f \colon X \to Y$ a continuous function, then
the mapping cylinder of $f$ is the attachment space
the mapping cone of $f$ is the attachment space
accordingly if $f \colon X \to Y$ is a continuous function between pointed spaces which preserves the basepoint, then the analogous construction with the reduced cylinder and the reduce cone, respectively, yield the reduced mapping cyclinder and the reduced mapping cone.
We now say this again in terms of pushouts:
For $f \colon X \longrightarrow Y$ a continuous function between pointed spces, its reduced mapping cone is the space
in the colimiting diagram
where $Cyl(X)$ is the reduced cylinder from def. 4.
The colimit appearing in the definition of the reduced mapping cone in def. 3 is equivalent to three consecutive pushouts:
The two intermediate objects appearing here are called
the plain reduced cone $Cone(X) \coloneqq \ast \underset{X}{\sqcup} Cyl(X)$;
the reduced mapping cylinder $Cyl(f) \coloneqq Cyl(X) \underset{X}{\sqcup} Y$.
Let $X \in Top^{\ast/}$ be any pointed topological space.
The mapping cone, def. 1, of $X \to \ast$ is called the reduced suspension of $X$, denoted
Via prop. 1 this is equivalently the coproduct of two copies of the cone on $X$ over their base:
This is also equivalently the cofiberf of $(i_0,i_1)$, hence (example 1) of the wedge sum inclusion:
The reduced suspension objects (def. 4) induced from the standard reduced cylinder $(-)\wedge (I_+)$ of example 4 are isomorphic to the smash product (def. 2) with the [[circle] (the 1-sphere)
For $f \colon X \longrightarrow Y$ a morphism in Top, then its unreduced mapping cone with respect to the standard cylinder object $X \times I$ def. \ref{TopologicalInterval}, is isomorphic to the reduced mapping cone, of the morphism $f_+ \colon X_+ \to Y_+$ (with a basepoint adjoined) with respect to the standard reduced cylinder:
By example 3, $Cone(f_+)$ is given by the colimit in $Top$ over the following diagram:
We may factor the vertical maps to give
This way the top part of the diagram (using the pasting law to compute the colimit in two stages) is manifestly a cocone under the result of applying $(-)_+$ to the diagram for the unreduced cone. Since $(-)_+$ is itself given by a colimit, it preserves colimits, and hence gives the partial colimit $Cone'(f)_+$ as shown. The remaining pushout then contracts the remaining copy of the point away.
Most of the relevant constructions on pointed topological spaces are immediate specializations of the general construction discussed at pointed object.