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 $X$ is said to be locally contractible if it has a basis of open subsets that consists of contractible topological spaces $U \hookrightarrow X$.
Sometimes one requires just that the inclusions $U \to X$ are null-homotopic map?s. This might be called semi-locally contractible.
One could also consider a basis of open sets such that the opens $U$ have (just) trivial homotopy groups, but this does not seem to crop up in practice.
A locale $X$ is locally contractible if, viewing a locale as a $(0,1)$-topos and hence a (very special kind of) $(\infty,1)$-topos, it is locally ∞-connected.
Is this right? Do these two definitions correspond in that a sober space or topological locale is locally contractible as a topological space iff it's locally contractible as a locale? —Toby
Any CW-complex is locally contractible (see there).
Any paracompact manifold is locally contractible.
Any contractible space is semi-locally contractible
The cone on the Hawaiian earring space is contractible, hence semi-locally contractible, but is not locally contractible, as any neighbourhood of the ‘bad point’ is not simply connected.
For $X$ a locally contractible topological space, the (∞,1)-category of (∞,1)-sheaves $Sh_{(\infty,1)}(X)$ is a locally ∞-connected (∞,1)-topos.
This is discussed at locally ∞-connected (∞,1)-site.
If one considers fundamental ∞-groupoids, the inclusion $U \to X$ being null-homotopic is equivalent to the induced (∞,1)-functor $\Pi(U) \to \Pi(X)$ being naturally isomorphic to the trivial functor sending everything to a single point.
David Roberts: The following may be straightforwardly obvious, but I have couched it as a conjecture, because I haven’t seen it in print.
If the space $X$ is semi-locally contractible then every locally constant $n$-stack on the site of open sets of $X$ is locally trivial.
See also locally ∞-connected (∞,1)-topos. There a converse to this conjecture is stated:
Let $C$ be a site coming from a coverage such that constant (∞,1)-presheaves satisfy descent over objects of $C$ with respect to the generating covering families. Then the (∞,1)-category of (∞,1)-sheaves $\mathbf{H} = Sh_{(\infty,1)}(C)$ is a locally ∞-connected (∞,1)-topos.
Last revised on September 18, 2021 at 10:18:37. See the history of this page for a list of all contributions to it.