Overview

• We construct a fully verified biequivalence between

  1. the (2,1)-category of coherent 2-groups
  2. the (2,1)-category of 2-truncated connected pointed types.

  We also derive a verified equality between them by way of univalence.

  A preprint outlining our mechanization is located here.

• We mechanize Owen Milner's equivalence between the type of n-groups (i.e., pointed connected n-types) and that of Sính triples (where n > 1).

The code has been checked with Agda 2.6.4.3.

Organization

The library has three main components, each component depending on the previous ones. It also has a fourth component collecting the main theorems.

• HoTT-Agda/

  A stripped down version of Andrew Swan's HoTT-Agda branch, with many changes and additions motivated by our construction of the biequivalence and by Milner's equivalence.

  See HoTT-Agda/README.md for details and for the license of the work inside this directory.

• Bicats/

  A collection of basic notions and facts about (2,1)-categories, which we also call bicategories in this work.

  See Bicats/README.md for details and for the license of the work inside this directory.

• Two-groups/

  Our formalization of the biequivalence and the induced equality.

  See Two-groups/README.md for details and for the license of the work inside this directory.

• Sinh/

  Our formalization of Owen Milner's proof of equivalence between pointed connected n-types (n > 1) and Sính triples.

  See Sinh/README.md for details and for the license of the work inside this directory.

• Final/

  A single file containing the final biequivalence and equality, Milner's equivalence along with a description of the group action produced by the equivalence, and the composite type equivalence between pointed connected 2-types and Sính triples.

  See Final/README.md for details and for the license of the work inside this directory.

Type-checking

We have successfully tested the following Docker container on Linux with 16 GB of RAM and 100 GB of swap space.

1. Build Docker image:

   docker build . -t 2group

   Our machine uses as much as 28.7 GB of physical memory and takes about 8 hours to build the image.

2. Generate HTML files:

   mkdir -p ./html
   docker run --mount type=bind,source=./html,target=/Two-groups/html 2group

   This may take a few minutes. The HTML files will be under html/, and html/Final-thms.agda.html will be the entry point.

If you can avoid the overhead of Docker, we suggest that you do so even if you have lots of available RAM.

We have found that type-checking directly on a MacOS with an M1 chip is much faster (see Final/README.md).

Important: Comment out the final two imports in Final/Final-thms to reduce the type-checking by over an hour. Doing so will check all relevant type equivalences but not the biequivalence.

Acknowledgement

This material is based upon work supported by the Air Force Office of Scientific Research under award number FA9550-21-1-0009. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the United States Air Force.