# nLab synthetic topology

## Idea

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 of 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.

## Dictionary

Martin Escardo has given the following translations between the two fields;

general topologytype theory
spacetype
continuous functionfunction
clopen setdecidable set
open setsemi-decidable set
closed setset with semi-decidable complement
discrete spacetype with decidable equality
Hausdorff spacetype with semi-decidable inequality
convergent sequencemap out of $\mathbb{N}_\infty$ (see below)
compact setexhaustively 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 hset?, whereas discrete spaces don’t need to be hsets).

## Semantics

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 infinity non-decreasing binary sequences) really does get interpreted semantically as the “generic convergent sequence”.

## Implementation

Many of the results that have originated from this view have been implemented in an Agda library

## References

Last revised on March 1, 2017 at 16:57:02. See the history of this page for a list of all contributions to it.