# Category Archives: Uncategorized

## The Brunerie Number Is -2

Since being introduced to HoTT around 3 years ago, I have, like so many others before me, developed a healthy(?) obsession with the Brunerie number (i.e. the number such that , as defined in Guillaume Brunerie’s PhD thesis). Over the … Continue reading

## On the ∞-topos semantics of homotopy type theory￼￼

Video and lecture notes are now available for a series of talks that took place last month at the Logic and Higher Structures workshop at CIRM-Luminy with the following abstract: Many introductions to homotopy type theory and the univalence axiom … Continue reading

## The HoTT Game

What is the HoTT Game? The Homotopy Type Theory (HoTT) Game is a project written by mathematicians for mathematicians interested in HoTT and no experience in proof verification, with the aim of introducing cubical agda as a tool for trying … Continue reading

## Postdoctoral position in Higher Category Theory at Johns Hopkins

The Department of Mathematics at Johns Hopkins University solicits applications for one two-year postdoctoral fellowship beginning Summer 2021 (with some flexibility in the start and end dates). The position is funded by the Army Research Office (ARO) through the Multidisciplinary … Continue reading

## Can the type of truth values be given the structure of a group?

When attempting to prove a theorem, not discussed in this post, I tried to give a certain set the structure of a group. After failing repeatedly, my proof attempts led me to consider a miniature version of the problem: can … Continue reading

## Postdoctoral position in HoTT at Johns Hopkins University

The Department of Mathematics at Johns Hopkins University solicits applications for one two-year postdoctoral fellowship beginning Summer 2021 (with some flexibility in the start and end dates). The position is funded by the Air Force Office of Scientific Research (AFOSR) … Continue reading

## Erik Palmgren Memorial Conference

Erik Palmgren, professor of Mathematical Logic at Stockholm University, passed away unexpectedly in November 2019. This conference will be an online workshop to remember and celebrate Erik’s life and work. There will be talks by Erik’s friends and colleagues, as … Continue reading

## HoTT 2019 Last Call

Last call for submissions INTERNATIONAL CONFERENCE AND SUMMER SCHOOL ON HOMOTOPY TYPE THEORY 12-17 August 2019 Carnegie Mellon University, Pittsburgh USA https://hott.github.io/HoTT-2019 Submission of talks and registration are open for the International Homotopy Type Theory conference (HoTT 2019), to be … Continue reading

## Introduction to Univalent Foundations of Mathematics with Agda

I am going to teach HoTT/UF with Agda at the Midlands Graduate School in April, and I produced lecture notes that I thought may be of wider use and so I am advertising them here. The source files I used … Continue reading

## Cubical Agda

Last year I wrote a post about cubicaltt on this blog. Since then there have been a lot of exciting developments in the world of cubes. In particular there are now two cubical proof assistants that are currently being developed … Continue reading

## Differential Geometry in Modal HoTT

As some of you might remember, back in 2015 at the meeting of the german mathematical society in Hamburg, Urs Schreiber presented three problems or “exercises” as he called it back then. There is a page about that on the … Continue reading

## A self-contained, brief and complete formulation of Voevodsky’s univalence axiom

I have often seen competent mathematicians and logicians, outside our circle, making technically erroneous comments about the univalence axiom, in conversations, in talks, and even in public material, in journals or the web. For some time I was a bit … Continue reading

## Impredicative Encodings of Inductive Types in HoTT

I recently completed my master’s thesis under the supervision of Steve Awodey and Jonas Frey. A copy can be found here. Known impredicative encodings of various inductive types in System F, such as the type of natural numbers do not … Continue reading

## A hands-on introduction to cubicaltt

Some months ago I gave a series of hands-on lectures on cubicaltt at Inria Sophia Antipolis that can be found at: https://github.com/mortberg/cubicaltt/tree/master/lectures The lectures cover the main features of the system and don’t assume any prior knowledge of Homotopy Type … Continue reading

## Type theoretic replacement & the n-truncation

This post is to announce a new article that I recently uploaded to the arxiv: https://arxiv.org/abs/1701.07538 The main result of that article is a type theoretic replacement construction in a univalent universe that is closed under pushouts. Recall that in … Continue reading

## HoTT MRC

From June 4 — 10, 2017, there will be a workshop on homotopy type theory as one of the AMS’s Mathematical Research Communities (MRCs).

## HoTT is not an interpretation of MLTT into abstract homotopy theory

Almost at the top of the HoTT website are the words: “Homotopy Type Theory refers to a new interpretation of Martin-Löf’s system of intensional, constructive type theory into abstract homotopy theory. ” I think it is time to change these words … Continue reading

## The HoTT Book does not define HoTT

The intent of this post is to address certain misconceptions that I’ve noticed regarding the HoTT Book and its role in relation to HoTT. (At least, I consider them misconceptions.) Overall, I think the HoTT Book project has been successful … Continue reading

## The cumulative hierarchy of sets (guest post by Jeremy Ledent)

In section 10.5 of the HoTT book, the cumulative hierarchy V is defined as a rather non-standard higher inductive type. We can then define a membership relation ∈ on this type, such that (V, ∈) satisfies most of the axioms … Continue reading

## Homotopical Patch Theory

This week at ICFP, Carlo will talk about our paper: Homotopical Patch Theory Carlo Angiuli, Ed Morehouse, Dan Licata, Robert Harper Homotopy type theory is an extension of Martin-Loef type theory, based on a correspondence with homotopy theory and higher category … Continue reading

## Homotopy Theory in Homotopy Type Theory: Introduction

Many of us working on homotopy type theory believe that it will be a better framework for doing math, and in particular computer-checked math, than set theory or classical higher-order logic or non-univalent type theory. One reason we believe this … Continue reading

## On Heterogeneous Equality

(guest post by Jesse McKeown) A short narative of a brief confusion, leading to yet-another-reason-to-think-about-univalence, after which the Author exposes his vaguer thinking to derision. The Back-story In the comments to Abstract Types with Isomorphic Types, Dan Licata mentioned O(bservational)TT, … Continue reading

## Abstract Types with Isomorphic Types

Here’s a cute little example of programming in HoTT that I worked out for a recent talk. Abstract Types One of the main ideas used to structure large programs is abstract types. You give a specification for a component, and … Continue reading

## The Simplex Category

I recently had occasion to define the simplex category inside of type theory. For some reason, I decided to use an inductive definition of each type of simplicial operators , rather than defining them as order-preserving maps of finite totally … Continue reading

## HoTT Updates

Many updates have been made to the various other pages on the site: Code, Events, Links, People, References. For example, there are several new items on models of the Univalence Axiom on the References page.

## Univalence versus Extraction

From a homotopical perspective, Coq’s built-in sort Prop is like an undecided voter, wooed by both the extensional and the intensional parties. Sometimes it leans one way, sometimes the other, at times flirting with inconsistency.

## Strong functional extensionality from weak

It’s amazing what you can find in the HoTT repository these days! I was browsing it the other week, looking up something quite different, when I came across a theorem in Funext.v (originally by Voevodsky) which answers, in a surprising … Continue reading

## HoTT in prose

I have adapted some basic HoTT theorems and proofs to prose form, in an attempt to better understand the results and their proofs. The Coq proof scripts often obscure details of the exposition, like the choice of fibration in an induction tactic, … Continue reading

## Axiomatic cohesion in HoTT

This post is to alert the members of the HoTT community to some exciting recent developments over at the n-Category Cafe. First, some background. Some of us (perhaps many) believe that HoTT should eventually be able to function as the … Continue reading

## What’s Special About Identity Types

From a homotopy theorist’s point of view, identity types and their connection to homotopy theory are perfectly natural: they are “path objects” in the category of types. However, from a type theorist’s point of view, they are somewhat more mysterious. … Continue reading