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
symmetric monoidal (∞,1)-category of spectra
Condensed sets are basic objects in condensed mathematics, whose aim is to provide a convenient setting in the framework for working with algebraic objects that are equipped with sort of a topology. A related alternative is provided by pyknotic sets.
Topological spaces formalize the idea of spaces with a notion of “nearness” of points. However, they fail to handle the idea of “points that are infinitely near, but distinct” in a useful way. Condensed sets handle this idea in a useful way. (Scholze 21)
For instance, $\mathbb{R}/\mathbb{Q}$ and $\ell^2(\mathbb{N})/\ell^1(\mathbb{N})$ are indiscrete as topological spaces but retain structure as condensed sets.
Condensed sets contain information about limits of images of any set, $A$, along the ultrafilters of $A$.
A condensed set is a sheaf of sets on the pro-étale site of a point — in other words, on the category of profinite spaces with finite jointly surjective families of maps as covers — that is the colimit of a small diagram of representables (a small sheaf?).
That is, a condensed set is a functor
such that the natural maps
and
are bijections for any profinite sets $S$ and $S'$, whereas the natural fork
is an equalizer for any surjection of profinite sets $S'\to S$.
Scholze, p.7 modifies this definition to deal with size issues:
For any uncountable strong limit cardinal $\kappa$, the category of $\kappa$-condensed sets $CondSet_\kappa$ is the category of sheaves on the site of profinite sets of cardinality less than $\kappa$, with finite jointly surjective families of maps as covers.
The category of condensed sets $CondSet$ is then the (large) colimit of the category of $\kappa$-condensed sets along the filtered poset of all uncountable strong limit cardinals $\kappa$, hence is the category of small sheaves?.
Several different sites can be used to define condensed sets, and, more generally, condensed ∞-groupoids:
Stone spaces (compact Hasdorff totally disconnected spaces);
Stonean spaces (compact Hausdorff extremally disconnected spaces).
In all three cases, morphisms are given by continuous maps and covering families are given by finite families of jointly surjective continuous maps.
The equivalence of sites is established in Yamazaki. See also Proposition 2.3, 2.7.
Condensed sets form a locally small, well-powered, locally cartesian closed infinitary-pretopos $CondSet$, that is neither a Grothendieck topos nor an elementary topos – since it lacks both a small separator (indeed, it is not even total) and a subobject classifier. It has a large separator of finitely presentable projectives, and hence is algebraically exact. (Campbell 20)
According to Peter Scholze in this comment on the nCafé and Mike Shulman in this comment on the nCafé, condensed sets satisfy external CoSHEP and WISC, but internal CoSHEP fails.
See Proposition 1.7 for the following proposition.
The forgetful functor from the category of topological spaces to κ-condensed sets is a faithful functor. It becomes fully faithful when restricted to κ-compactly generated spaces.
This functor admits a left adjoint, which sends a condensed set $T$ to the topological space given by the underlying set $T(*)$ of $T$ equipped with the quotient topology induced by the map
where $S$ runs over all (κ-small) profinite sets mapping into $T$. The counit of this adjunction coincides with the counit $X^{cg}\to X$ of the adjunction between (κ-small) compactly generated spaces and topological spaces.
The left adjoint exists also for topological groups and other algebraic structures, but in this case, the underlying set is not $T(*)$.
The equivalence of various sites for condensed sets is established in
Last revised on May 20, 2023 at 12:19:14. See the history of this page for a list of all contributions to it.