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
Brouwer‘s fixed point theorem says that every continuous function from a compact convex set to itself has at least one fixed point.
This is also a special case of the Lefschetz fixed point theorem, see there.
Brouwer‘s fixed point theorem is provable in toposes other than the category of sets:
Brouwer‘s fixed point theorem holds in the parameterized realizability topos constructed in Bauer & Hanson 2024.
Brouwer‘s fixed point theorem holds in the internal language of the topos of light condensed sets (see Cherubini, Coquand, Geerligs & Moeneclaey 2024).
Textbook account
See also
Discussion of the Brouwer’s fixed-point theorem in toposes other than the category of sets:
Felix Cherubini, Thierry Coquand, Freek Geerligs, Hugo Moeneclaey, A Foundation for Synthetic Stone Duality (arXiv:2412.03203)
Andrej Bauer, James Hanson, The Countable Reals (arXiv:2404.01256)
Discussion in cohesive homotopy type theory is in
Last revised on April 17, 2026 at 01:57:51. See the history of this page for a list of all contributions to it.