nLab simplicial type theory

Idea

Simplicial type theory (Riehl & Shulman 2017, §3) is a homotopy type theory which introduces additional non-fibrant layers on top of Martin-Löf type theory to provide a logical calculus for a directed interval type and with it for geometric shapes, consisting of Segal space-types, Rezk complete-types, and covariant fibrations.

Formal presentations

There are many different ways to formalize the directed interval primitive in simplicial type theory:

• one could simply postulate via inference rules and axioms a directed interval primitive in a separate layer.

• one could use two-level type theory consisting of a non-fibrant layer, and then axiomatise a directed interval primitive in the non-fibrant layer

• one could use type theory with shapes consisting of a non-fibrant layer of cubes which is a simple type theory and a non-fibrant layer of topes which is a propositional logic over the cube layer, and then define the directed interval primitive using the cube and tope layers

Two-level type theory

We use $\Xi$ to represent the context in the non-fibrant layer, and $\Gamma$ to represent the context in the fibrant layer. Type judgments in the non-fibrant layer are represented by $A \; \mathrm{exotype}$, while type judgments in the fibrant layer are just represented by $A \; \mathrm{type}$.

Directed interval primitive

The directed interval primitive $\mathbb{2}$ in simplicial type theory is a non-trivial bounded total order. Bounded total orders have many definitions in mathematics, such as a bounded partial order satisfying totality, or a lattice satisfying totality. As such, there are multiple possible sets of inference rules one could use to present the directed interval in the two-level type theory formalization.

The inference rules for the directed interval primitive $(\mathbb{2}, \leq)$ as a partial order in the two-level type theory formalization are as follows:

$$\frac{\Xi \; \mathrm{ctx}}{\Xi \vdash \mathbb{2} \; \mathrm{exotype}} \qquad \frac{\Xi \; \mathrm{ctx}}{\Xi \vdash 0:\mathbb{2}} \qquad \frac{\Xi \; \mathrm{ctx}}{\Xi \vdash 1:\mathbb{2}} \qquad \frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2}, y:\mathbb{2} \vdash x \leq y \; \mathrm{exotype}} \qquad \frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2}, y:\mathbb{2} \vdash \tau_\leq(x, y):\mathrm{isProp}(x \leq y)}$$

$$\frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2} \vdash \mathrm{refl}_\leq(x):x \leq x} \qquad \frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2}, y:\mathbb{2}, z:\mathbb{2}, p:x \leq y, q:y \leq z \vdash \mathrm{trans}_\leq(x, y, z, p, q):(x \leq z)}$$

$$\frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2}, y:\mathbb{2} \vdash \mathrm{antisym}_\leq(x, y):\mathrm{isEquiv}(idtoleqgeq(x, y))} \quad \mathrm{where} \quad \begin{array}{l} idtoleqgeq(x, y):(x =_\mathbb{2} y) \to (x \leq y) \times (y \leq x) \\ idtoleqgeq(x, x)(\mathrm{id}_\mathbb{2}(x)) \coloneqq (\mathrm{refl}_\leq(x), \mathrm{refl}_\leq(x)) \end{array}$$

$$\frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2}, y:\mathbb{2} \vdash \mathrm{totl}_\leq(x, y):[(x \leq y) + (y \leq x)]} \qquad \frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2} \vdash \mathrm{bot}_\leq(x):0 \leq x} \qquad \frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2} \vdash \mathrm{top}_\leq(x):x \leq 1}$$

$$\frac{\Xi \; \mathrm{ctx}}{\Xi \vdash \mathrm{nontriv}_\leq:(0 =_\mathbb{2} 1) \to \mathbb{0}}$$

The inference rules for the directed interval primitive $\mathbb{2}$ as a lattice in the two-level type theory formalization are as follows:

$$\frac{\Xi \; \mathrm{ctx}}{\Xi \vdash \mathbb{2} \; \mathrm{exotype}} \qquad \frac{\Xi \; \mathrm{ctx}}{\Xi \vdash \tau_0:\mathrm{isSet}(\mathbb{2})} \qquad \frac{\Xi \; \mathrm{ctx}}{\Xi \vdash 0:\mathbb{2}} \qquad \frac{\Xi \; \mathrm{ctx}}{\Xi \vdash 1:\mathbb{2}} \qquad \frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2}, y:\mathbb{2} \vdash x \wedge y:\mathbb{2}} \qquad \frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2}, y:\mathbb{2} \vdash x \vee y:\mathbb{2}}$$

$$\frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2} \vdash \mathrm{idem}_\wedge(x):x \wedge x =_{\mathbb{2}} x} \qquad \frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2}, y:\mathbb{2} \vdash \mathrm{comm}_\wedge(x, y):x \wedge y =_{\mathbb{2}} y \wedge x}$$

$$\frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2}, y:\mathbb{2}, z:\mathbb{2} \vdash \mathrm{assoc}_\wedge(x, y, z):(x \wedge y) \wedge z =_{\mathbb{2}} x \wedge (y \wedge z)}$$

$$\frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2} \vdash \mathrm{lunit}_\wedge(x):1 \wedge x =_{\mathbb{2}} x} \qquad \frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2} \vdash \mathrm{runit}_\wedge(x):x \wedge 1 =_{\mathbb{2}} x}$$

$$\frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2} \vdash \mathrm{idem}_\vee(x):x \vee x =_{\mathbb{2}} x} \qquad \frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2}, y:\mathbb{2} \vdash \mathrm{comm}_\vee(x, y):x \vee y =_{\mathbb{2}} y \vee x}$$

$$\frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2}, y:\mathbb{2}, z:\mathbb{2} \vdash \mathrm{assoc}_\vee(x, y, z):(x \vee y) \vee z =_{\mathbb{2}} x \vee (y \vee z)}$$

$$\frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2} \vdash \mathrm{lunit}_\vee(x):0 \vee x =_{\mathbb{2}} x} \qquad \frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2} \vdash \mathrm{runit}_\vee(x):x \vee 0 =_{\mathbb{2}} x}$$

$$\frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2}, y:\mathbb{2} \vdash \mathrm{asorp}_\wedge(x, y):x \wedge (x \vee y) =_{\mathbb{2}} x} \qquad \frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2}, y:\mathbb{2} \vdash \mathrm{asorp}_\vee(x, y):x \vee (x \wedge y) =_{\mathbb{2}} x}$$

$$\frac{\Xi \; \mathrm{ctx}}{\Xi, x:\mathbb{2}, y:\mathbb{2} \vdash \mathrm{totl}(x, y):[(x \wedge y =_{\mathbb{2}} x) + (x \wedge y =_{\mathbb{2}} y)]} \quad \frac{\Xi \; \mathrm{ctx}}{\Xi \vdash \mathrm{nontriv}_\leq:(0 =_\mathbb{2} 1) \to \mathbb{0}}$$

Simplices and subshapes

We inductively define the $n$-simplices as the following family of non-fibrant types:

• The $0$-simplex is the unit type in the non-fibrant layer

  $$\Delta^0 \coloneqq \mathbf{1}$$

• In the non-fibrant layer, let $\mathrm{Fin}(n) \coloneqq \sum_{i:\mathbb{N}} i \lt n$ be the canonical finite type with $n$ elements. The $n+1$-simplex is defined as the type

  $$n:\mathbb{N} \vdash \Delta^{n + 1} \coloneqq \sum_{t:\mathrm{Fin}(n + 1) \to \mathbb{2}} \prod_{i:\mathrm{Fin}(n)} t(\pi_1(i + 1)) \leq t(\pi_1(i))$$

The first few simplicies can be explicitly written as

$$\Delta^1 \coloneqq \mathbb{2}$$

$$\Delta^2 \coloneqq \sum_{t:\mathrm{Fin}(2) \to \mathbb{2}} t(1) \leq t(0)$$

$$\Delta^3 \coloneqq \sum_{t:\mathrm{Fin}(3) \to \mathbb{2}} (t(2) \leq t(1)) \times (t(1) \leq t(0))$$

Subshapes of the simplicies could also be defined.

• The boundary of the $1$-simplex is defined as

  $$\partial \Delta^1 \coloneqq \sum_{t:\Delta^1} [(t =_{\Delta^1} 0) + (t =_{\mathbb{2}} 1)]$$

• The boundary of the $2$-simplex is defined as

  $$\partial \Delta^2 \coloneqq \sum_{t:\mathrm{Fin}(2) \to \mathbb{2}} [(t(0) =_{\mathbb{2}} 0) \times (t(0) \leq t(1)) + (t(0) =_{\mathbb{2}} t(1)) + (t(0) \leq t(1)) \times (t(1) =_{\mathbb{2}} 1)]$$

Type theory with shapes

…

See also

• Reedy model structure

• bisimplicial set

• synthetic $(\infty,1)$-category theory

• type theory with shapes

• two-level type theory

References

• Emily Riehl, Mike Shulman, A type theory for synthetic ∞-categories, Higher Structures 1 1 (2017) [arxiv:1705.07442, published article]

• Jonathan Weinberger, A Synthetic Perspective on $(\infty,1)$-Category Theory: Fibrational and Semantic Aspects $[$arXiv:2202.13132$]$

• Jonathan Weinberger, Strict stability of extension types $[$arXiv:2203.07194$]$

A talk on synthetic (infinity,1)-category theory in simplicial type theory and infinity-cosmos theory:

• Emily Riehl, The synthetic theory of infinity-categories vs the synthetic theory of infinity-categories, Homotopy Type Theory Electronic Seminar Talks, 1 March 2018, (video, slides)

A proof assistant implementing simplicial type theory:

• Rzk[github.com/fizruk/rzk]

Formalization of the $(\infty,1)$-Yoneda lemma via simplicial homotopy type theory (in Rzk):

• Nikolai Kudasov, Emily Riehl, Jonathan Weinberger. Formalizing the $\infty$-categorical Yoneda lemma (2023) [arXiv:2309.08340]