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 extremally disconnected if the closure of any open subset is still an open subset, or, equivalently, the closures of disjoint open subsets are disjoint.
The extremally disconnected spaces can be characterized among the Hausdorff spaces by the fact that any continuous function mapping a dense subspace of an extremally disconnected space to a compact Hausdorff space, can be extended to the whole space. They can also be characterized among the completely regular spaces by the fact that the lattice of continuous functions mapping an extremally dis connected space to the unit interval is complete (cf. (Giliman-Jerison, 1960), 3N and 6M). (Strauss 1967)
(Gleason 1958)
Extremally disconnected topological spaces are precisely the projective objects in the category of compact Hausdorff topological spaces, and in fact in the category of regular Hausdorff topological spaces and proper maps between them.
See e.g. (Bhatt-Scholze 13, below theorem 1.8). For the second part see (Strauss 1967).
For $R$ a w-contractible ring, the profinite set $\pi_0(Spec R)$ is an extremally disconnected profinite set.
Part of (Bhatt-Scholze 13, theorem 1.8).
The extremal disconnectedness of a space is correlated with the property that the frame of its open subsets is a De Morgan Heyting algebra hence with the validity of the De Morgan law in logic, since a result by Johnstone says that a topos $Sh(X)$ of sheaves on a topological space $X$ is a De Morgan topos precisely when $X$ is extremally disconnected and this implies that all subobject lattices $sub(X)$ are De Morgan Heyting algebras, in particular $sub(1)$ corresponding to the frame of opens of $X$ (cf. De Morgan topos for details and references).
Being extremally disconnected is equivalently a lifting property with respect to a surjective proper morphism of finite topological spaces (for more see the discussion at separation axioms in terms of lifting properties):
Namely that a topological space $X$ is extremally disconnected means equivalently that it solves the following lifting problems (from here):
Here boxes indicate open subsets in a finite topological space, and an arrow $x\to c$ means that $c$ is in the closure of $x$. See at Background and notation for more.
One may deduce Gleason’s theorem (Prop. ) in terms of these lifting properties:
Both being surjective and being proper are right lifting properties. Hence, the existence of a weak factorization system generated by this morphism implies that each space admits a surjective proper map from an extremally disconnected space.
For more see at separation axioms in terms of lifting properties the section extremally disconnected spaces being projective.
For $S$ any set regarded as a discrete topological space, its Stone-Cech compactification is extremally disconnected.
(e.g. Bhatt-Scholze 13, example 2.4.6)
The profinite set underlying the p-adic integers, regarded as a Stone space, is not extremally disconnected.
(e.g. Bhatt-Scholze 13, example 2.4.7)
The result on projective spaces stems from
Andrew M. Gleason, Projective topological spaces, Ill. J. Math. 2 no.4A (1958) pp.482-489. (euclid:ijm/1255454110)
D. P. Strauss, (1967). Extremally Disconnected Spaces. Proceedings of the American Mathematical Society, 18(2), 305. doi:10.2307/2035286
Textbook account:
L.Giliman, M.Jerison. Rings of continuous functions. Van Nostrand, 1960.
Discussion in the context of the pro-etale site:
Last revised on October 17, 2021 at 14:33:11. See the history of this page for a list of all contributions to it.