nLab formal (infinity,1)-category theory

Idea

Formal $(\infty,1)$-category theory is to $(\infty,1)$-category theory what formal category theory is to 1-category theory, that is: a synthetic approach to $(\infty,1)$-categories standing in relation to synthetic homotopy theory as $(\infty, 1)$-category theory stands to homotopy theory. Consequently, formal $(\infty,1)$-category theory has also been called synthetic $(\infty,1)$-category theory.

Approaches

There have been two main styles of approaches to formal $(\infty,1)$-category theory: categorical, which are $(\infty,2)$-categorical in nature; and type theoretic, which propose deductive systems for reasoning about $(\infty,1)$-categories.

Category theoretic approaches

• Following Joyal 2008 p. 158, Riehl & Verity 2013 observe that the homotopy 2-category of $(\infty,1)$-categories $2Ho\big( Cat_{(\infty,1)}\big)$ retains much of the interesting structure of the $(\infty,2)$-category $Cat_{(\infty,1)}$. Thus, one may use the techniques of formal category theory to study certain aspects of the formal theory of $(\infty,1)$-categories. For instance, this includes the theory of adjoint $(\infty,1)$-functors (which are automatically homotopy coherent), but not the theory of $(\infty,1)$-monads (which are not). This is developed in Elements of ∞-Category Theory in the setting of a virtual equipment.

• For presentable $(\infty,1)$-categories, formal $(\infty,1)$-category theory is directly accessible from “abstract homotopy theory” in the sense of combinatorial model category-theory, in that $2Ho\big( PresCat_{(\infty,1)}\big) \,\simeq\,$ 2Ho(CombModCat) (Pavlov 2021).

• Ruit 2023 has introduced an $\infty$-double categorical generalisation of fibrant double categories.

• There is also the approach by Cisinski et al.

Type theoretic approaches

The type theoretic approaches attempt to formalise the internal logic of a cohesive $(\infty,1)$-topos or local $(\infty,1)$-topos $\mathbf{H}$ over $\infty\mathrm{Grpd}$, such that $(\infty,1)\mathrm{Cat} \hookrightarrow \mathbf{H}$ is a $(\infty,1)$-subcategory of $\mathbf{H}$. Examples include

• Riehl & Shulman 2017 propose a type theory called simplicial type theory, which have categorical semantics in simplicial infinity-groupoids or bisimplicial sets. This is further developed by Buchholtz & Weinberger 2021.

• Gratzer, Weinberger & Buchholtz 2024 propose a modification of simplicial type theory called triangulated type theory, which have categorical semantics in cubical infinity-groupoids or cubical-simplicial sets.

• Weaver & Licata 2020 propose a type theory, which has semantics in bicubical sets.

• Kolomatskaia & Shulman 2023 propose a type theory called displayed type theory, which has categorical semantics in augmented semi-simplicial $\infty$-groupoids. Unlike the previous three type theories, the semantics is not cohesive over $\infty\mathrm{Grpd}$ (Aberlé 2024).

Related pages

• formal category theory

• $(\infty,2)$-topos

• internal category in homotopy type theory

• synthetic mathematics, synthetic homotopy theory

• $(\infty,1)$-category theory

• directed homotopy type theory

• simplicial type theory, cubical type theory

References

• Denis-Charles Cisinski, Bastiaan Cnossen, Hoang Kim Nguyen, Tashi Walde, Formalization of Higher Categories, work in progress (pdf)

Category theoretic approaches

With (small) (∞,1)-categories modeled as quasi-categories, their homotopy 2-category was considered first in

• André Joyal, p. 158 in: The theory of quasicategories and its applications, lectures at Advanced Course on Simplicial Methods in Higher Categories, CRM 2008 (pdf, pdf)

and then developed further (in the generality of homotopy 2-categories of ∞-cosmoi) in:

• Emily Riehl, Dominic Verity, The 2-category theory of quasi-categories, Advances in Mathematics Volume 280, 6 August 2015, Pages 549-642 (arXiv:1306.5144, doi:10.1016/j.aim.2015.04.021)

• Emily Riehl, Dominic Verity, Infinity category theory from scratch, Higher Structures Vol 4, No 1 (2020) (arXiv:1608.05314, pdf)

• Emily Riehl, Dominic Verity, Elements of ∞-Category Theory, Cambridge studies in advanced mathematics 194, Cambridge University Press (2022) $[$doi:10.1017/9781108936880, ISBN:978-1-108-83798-9, pdf$]$

• Emily Riehl, The formal theory of ∞-categories, talk at Categories and Companions Symposium June 8–12, 2021 (video)

In an (∞,1)-category theoretic version of proarrow equipments:

• Jaco Ruit, Formal category theory in ∞-equipments I (arXiv:2308.03583)

Type theoretic approaches

For synthetic (infinity, 1)-category theory in cubical type theory via bicubical sets:

• Matthew Weaver, Daniel Licata, A Constructive Model of Directed Univalence in Bicubical Sets, in: Proceedings of the 35th Annual ACM/IEEE Symposium on Logic in Computer Science. LICS ’20, Association for Computing Machinery (2020) 915–928 [doi:10.1145/3373718.3394794]

Exposition in

• Matthew Weaver, A Constructive Model of Directed Univalence in Bicubical Sets, talk at HoTTEST (April 2020) [pdf, video]

For synthetic (infinity, 1)-category theory in simplicial type theory:

• Emily Riehl, Michael Shulman, A type theory for synthetic $\infty$-categories $[$arXiv:1705.07442$]$

• Ulrik Buchholtz, Jonathan Weinberger, Synthetic fibered $(\infty,1)$-category theory $[$arXiv:2105.01724, talk slides$]$

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

• Fredrik Bakke, Segal Spaces in Homotopy Type Theory. Master thesis $[$no.ntnu:inspera:99217069:14871483$]$

• César Bardomiano Martínez, Limits and colimits of synthetic $\infty$-categories $[$arXiv:2202.12386$]$

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

• Jonathan Weinberger, Two-sided cartesian fibrations of synthetic $(\infty,1)$-categories $[$arXiv:2204.00938$]$

• Jonathan Weinberger, Internal sums for synthetic fibered $(\infty,1)$-categories $[$arXiv:2205.00386$]$

• Daniel Gratzer, Jonathan Weinberger, Ulrik Buchholtz, Directed univalence in simplicial homotopy type theory (arXiv:2407.09146)

• Daniel Gratzer, Jonathan Weinberger, Ulrik Buchholtz, The Yoneda embedding in simplicial type theory (arXiv:2501.13229)

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)

The authors of the following paper suggest that it should be possible to formalise $(\infty,1)$-categories in displayed type theory:

• Astra Kolomatskaia, Michael Shulman, Displayed Type Theory and Semi-Simplicial Types [arXiv:2311.18781]

• C.B. Aberlé, Parametricity via Cohesion [arXiv:2404.03825]

For synthetic $(\infty,1)$-categories in synthetic domain theory:

• Jonathan Sterling, Baby steps in higher domain theory, talk at Homotopy Type Theory and Computing – Classical and Quantum, Center for Quantum and Topological Systems (video)

• Leoni Pugh, Jonathan Sterling, When is the partial map classifier a Sierpiński cone? (arXiv:2504.06789)