natural deduction metalanguage, practical foundations
type theory (dependent, intensional, observational type theory, homotopy type theory)
computational trinitarianism =
propositions as types +programs as proofs +relation type theory/category theory
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
Synthetic topology, like synthetic domain theory, synthetic differential geometry, and synthetic computability, are part of synthetic mathematics. It uses the internal logic of a topos to develop a part of mathematics. In this case topology. This is closely related to topology via logic and abstract Stone duality.
The formal system of type theory has semantics in many categories, and in particular in many categories of “spaces”. Thus types may be regarded not just as sets but as topological objects. Interestingly, a good deal of this “topology” can be detected intrinsically in type theory, often corresponding to the possible failure of principles of classical mathematics.
Martín Escardó has given the following translations between the two fields:
general topology | type theory |
---|---|
space | type |
continuous function | function |
clopen set | decidable set |
open set | semi-decidable set |
closed set | set with semi-decidable complement |
discrete space | type with decidable equality |
Hausdorff space | type with semi-decidable inequality |
convergent sequence | map out of $\mathbb{N}_\infty$ (see below) |
compact set | exhaustively searchable set, in a finite number of steps |
It should be stressed that the concepts on the right are not the only ways to represent the topological concepts on the left in type theory. For instance, in cohesive homotopy type theory there is a notion of “discrete space” that has nothing to do with decidable equality (in particular, in homotopy type theory a type with decidable equality is necessarily an h-set, whereas discrete spaces don’t need to be h-sets).
There are many different topological semantics for type theory, but one which seems especially closely related to the above dictionary is the topological topos. For instance, in that case the internally defined set $\mathbb{N}_\infty$ (the set of infinite decreasing binary sequences) really does get interpreted semantically as “the generic convergent sequence”.
Many of the results that have originated from this view have been implemented in an Agda library.
Martín Escardó, Synthetic topology of data types and classical spaces, 2004 (pdf)
Martín Escardó, The topology of seemingly impossible functional programs, 2012 (pdf, almost same presentation with minor changes pdf)
Andrej Bauer, Davorin Lešnik, Metric Spaces in Synthetic Topology, 2010 (pdf)
Davorin Lešnik, Synthetic Topology and Constructive Metric Spaces, PhD, 2010 (arXiv:2104.10399)
Davorin Lešnik, Unified Approach to Real Numbers in Various Mathematical Settings, 2014 (arXiv:1402.6645)
Steven Vickers, Locales and toposes as spaces, 2007 (pdf)
Last revised on December 28, 2022 at 02:08:00. See the history of this page for a list of all contributions to it.