nLab real-cohesive homotopy type theory

Idea

$\mathbb{R}$-cohesive homotopy type theory or real-cohesive homotopy type theory is a version of cohesive homotopy type theory which has the axiom of real cohesion. It provides a synthetic foundation for topology, real analysis, classical homotopy theory, and algebraic topology.

Presentation

We assume a spatial type theory presented with crisp term judgments $a::A$. In addition, we also assume the spatial type theory has a Dedekind real numbers type $\mathbb{R}$, and $\mathbb{R}$-localizations $\mathcal{L}_{\mathbb{R}}(-)$.

Given a type $A$, let us define $\mathrm{const}_{A, \mathbb{R}}:A \to (\mathbb{R} \to A)$ to be the type of all constant functions in the Dedekind real numbers $\mathbb{R}$:

$$\delta_{\mathrm{const}_{A, \mathbb{R}}}(a, r):\mathrm{const}_{A, \mathbb{R}}(a)(r) =_A a$$

There is an equivalence $\mathrm{const}_{A, 1}:A \simeq (1 \to A)$ between the type $A$ and the type of functions from the unit type $1$ to $A$. Given types $B$ and $C$ and a function $F:(B \to A) \to (C \to A)$, type $A$ is $F$-local if the function $F:(B \to A) \to (C \to A)$ is an equivalence of types.

A crisp type $\Xi \vert () \vdash A$ is discrete if the function $(-)_\flat:\flat A \to A$ is an equivalence of types.

The axiom of $\mathbb{R}$-cohesion states that for the crisp affine line $\Xi \vert () \vdash \mathbb{R} \; \mathrm{type}$, given any crisp type $\Xi \vert () \vdash A \; \mathrm{type}$, $A$ is discrete if and only if $A$ is $(\mathrm{const}_{A, 1}^{-1} \circ \mathrm{const}_{A, \mathbb{R}})$-local.

This allows us to define discreteness for non-crisp types: a type $A$ is discrete if $A$ is $(\mathrm{const}_{A, 1}^{-1} \circ \mathrm{const}_{A, \mathbb{R}})$-local.

The shape modality in $\mathbb{R}$-cohesive homotopy type theory is then defined as the $\mathbb{R}$-localization $\esh(A) \coloneqq \mathcal{L}_{\mathbb{R}}(A)$, which ensures that the shape of $\mathbb{R}$ itself is a contractible type.

 See also

• cohesive homotopy type theory
• axiom of real cohesion

References

• Mike Shulman, Brouwer’s fixed-point theorem in real-cohesive homotopy type theory, Mathematical Structures in Computer Science Vol 28 (6) (2018): 856-941 (arXiv:1509.07584, doi:10.1017/S0960129517000147)