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Here are the slides of my talk "The dawn of formalized mathematics" from the 8th European Congress of Mathematics, which is taking place online and in Protorož, Slovenia, from June 20 to 26, 2021:

Keynote presentation, viewable online in your browser. Turn on the speaker notes by clicking on the rectangular icon in the top-left corner. Slides with speaker notes (PDF). Unfortunately, Keynote does not make the hyperlinks active when exporting PDF. Video recording of the talk.→ continue reading(2 comments) Computing an integer using a Grothendieck topos18 May 2021Martin Escardo Constructive mathematics, Type theory, FormalizationA while ago, my former student Chuangjie Xu and I computed an integer using a sheaf topos. For that purpose,

we developed our mathematics constructively, we formalized our mathematics in Martin-Löf type theory, in Agda notation, we pressed a button, and after a few seconds we saw the integer we expected in front of us.Well, it was a few seconds for the computer in steps (3)-(4), but three years for us in steps (1)-(2).

→ continue reading The Burali-Forti argument in HoTT/UF22 February 2021Martin Escardo Type theory, Constructive mathematicsThis is joint work with Marc Bezem, Thierry Coquand, Peter Dybjer.

We use theBurali-Fortiargument to show that, in homotopy type theory and univalent foundations,the embedding $$ \mathcal{U} \to \mathcal{U}^+$$ of a universe$\mathcal{U}$ into its successor $\mathcal{U}^+$ is not anequivalence. We also establish this for the types of sets, magmas, monoids andgroups. The arguments in this post are also written inAgda.

→ continue reading Synthetic mathematics with an excursion into computability theory03 February 2021Andrej Bauer Talks, Synthetic computabilityIt is my pleasure to have the opportunity to speak at the University of Wisconsin Logicseminar. The hosts are graciously keeping the seminar open for everyone.I will speak about a favorite topic of mine:

Synthetic mathematics with an excursion into computability theorySpeaker: Andrej Bauer (University of Ljubljana)

Location: University of Wisconsin Logic seminar

Time: February 8th 2021, 21:30 UTC (3:30pm CST in Wisconsin, 22:30 CET in Ljubljana)

Video link: the Zoom link is available at the seminar page

Abstract:

According to Felix Klein, “synthetic geometry is that which studies figures as such, without recourse toformulae, whereas analytic geometry consistently makes use of such formulae as can be written down after the adoption ofan appropriate system of coordinates”. To put it less eloquently, the synthetic method axiomatizes geometry directly byconstruing points and lines as primitive notions, whereas the analytic method builds a model, the Euclidean plane, fromthe real numbers.

Do other branches of mathematics posses the synthetic method, too? For instance, what would “synthetic topology” looklike? To build spaces out of sets, as topologists usually do, is the analytic way. The synthetic approach must construespaces as primitive and axiomatize them directly, without any recourse to sets. It cannot introduce continuity as adesirable property of functions, for that would be like identifying straight lines as the non-bending curves.

It is indeed possible to build the synthetic worlds of topology, smooth analysis, measure theory, and computability. Ineach of them, the basic structure – topological, smooth, measurable, computable – is implicit by virtue of permeatingeverything, even logic itself. The synthetic worlds demand an economy of thought that the unaccustomed mind findsfrustrating at first, but eventually rewards it with new elegance and conceptual clarity. The synthetic method is stillfruitfully related to the analytic method by interpretation of the former in models provided by the latter.

We demonstrate the approach by taking a closer look at synthetic computability, whose central axiom states that thereare countably many countable subsets of the natural numbers. The axiom is validated and explained by its interpretationin the effective topos, where it corresponds to the familiar fact that the computably enumerable sets may be computablyenumerated. Classic theorems of computability may be proved in a straightforward manner, without reference to anynotion of computation. We end by showing that in synthetic computability Turing reducibility is expressed in terms ofsequential continuity of maps between directed-complete partial orders.

The slides and the videorecording of the talk are now available.

→ continue reading Every proof assistant: introducing homotopy.io – a proof assistant for geometrical higher category theory24 November 2020Andrej Bauer Talks, Every proof assistantAfter a short pause, our next talk in the series will be given by Jamie Vicary, who willpresent a proof assistant in which the proofs are drawn!

Introducing homotopy.io: A proof assistant for geometrical higher category theory→ continue reading A general definition of dependent type theories14 September 2020Andrej Bauer Publications, Type theoryTime: Thursday, November 26, 2020 from 15:00 to 16:00 (Central European Time, UTC+1)

Location: online at Zoom ID 989 0478 8985

Speaker: Jamie Vicary (University of Cambridge)

Proof assistant: homotopy.ioAbstract:

Weak higher categories can be difficult to work with algebraically,with the weak structure potentially leading to considerable bureaucracy.Conjecturally, every weak infty-category is equivalent to a"semistrict" one, in which unitors and associators are trivial; such asetting might reduce the burden of constructing largeproofs. In this talk, I will present the proof assistant homotopy.io,which allows direct construction of composites in a finitely-generatedsemistrict (infty,infty)-category. The terms of the proof assistanthave a geometrical interpretation as string diagrams, and interactionwith the proof assistant is entirely geometrical, by clicking anddragging with the mouse, completely unlike more traditional computeralgebra systems. I will give an outline of the underlying theoreticalfoundations, and demonstrate use of the proof assistant to constructsome nontrivial homotopies, rendered in 2d and 3d. I will close withsome speculations about the possible interaction of such a system withmore traditional type-theoretical approaches. (Joint work with LukasHeidemann, Nick Hu and David Reutter.)

References:

David Reutter, Jamie Vicary:High-level methods for homotopy construction inassociative n-categories, arXiv:1902.03831preprint, February 2019. homotopy.io at Lab

The preprint version of the paper A general definition of dependent typetheories has finally appeared on the arXiv! Over threeyears ago Peter Lumsdaine invited me to work on thetopic, which I gladly accepted, and dragged my student PhilippHaselwarter into it. We set out to give *an* answer tothe queation:

What is type theory, precisely?

At least for me the motivation to work on such a thankless topic came from VladimirVoevodsky, who would ask the question to type-theoretic audiences. Some took him to be atroll and others a newcomer who just had to learn more type theory. I was among thelatter, but eventually the question got through to me – I could point to any number of*specific* examples of type theories, but not a comprehensive and mathematically precisedefinition of the *general* concept.

It is too easy to dismiss the question by claiming that type theory is an open-ended conceptwhich therefore cannot be completely captured by any mathematical definition. Ofcourse it is open-ended, but it does not follow at all that we should not evenattempt to define it. If geometers were equally fatalistic about the open-ended notion ofspace we would never have had modern geometry, topology, sheaves – heck, half of 20thcentury mathematics would not be there!

Of course, we are neither the first nor the last to give a definition of type theory, andthat is how things should be. We claim no priority or supremacy over other definitions andviews of type theory. Our approach could perhaps be described as "concrete" and"proof-theoretic":

We wanted to*stay close to traditional syntax*. We gave a

*complete*and

*precise*definition. We aimed for a level of generality that allows useful meta-theory of a wide range of type theories.

One can argue each of the above points, and we have done so among ourselves many times.Nevertheless, I feel that we have accomplished something worthwhile – but the ultimatejudges will be our readers, or lack of them. You are kindly invited to take a look at thepaper.

Download PDF: `arxiv.org/pdf/2009.05539.pdf`

I should not forget to mention that Peter, with modest help from Philipp and me,formalized almost the entire paper in Coq! See the repository`general-type-theories`

atGitHub.

I am happy to announce that we are restarting the "Every proof assistants" series of talkswith Anders Mörtberg who will talk about Cubical Agda. Note that we are moving the seminartime to a more reasonable hour, at least as far as the working people in Europe areconcerned.

Cubical Agda: A Dependently Typed Programming Language with Univalence and Higher Inductive TypesTime: Thursday, September 17, 2020 from 15:00 to 16:00 (Central European Summer Time, UTC+2)

Location: online at Zoom ID 989 0478 8985

Speaker: Anders Mörtberg (Stockholm University)

Proof assistant: Cubical AgdaAbstract:The dependently typed programming language Agda has recently beenextended with a cubical mode which provides extensionality principlesfor reasoning about equality, such as function and propositionalextensionality. These principles are typically added axiomatically toproof assistants based on dependent type theory which disrupts theconstructive properties of these systems. Cubical type theory providesa solution by giving computational meaning to Homotopy Type Theory andUnivalent Foundations, in particular to the univalence axiom andhigher inductive types. In the talk I will discuss how Agda wasextended to a full-blown proof assistant with native support forunivalence and a general schema of higher inductive types. I will alsoshow a variety of examples of how to use Cubical Agda in practice toreason about mathematics and computer science.

The talk video recording is available.

We have more talks in store, but we will space them out a bit to give slots to our local seminar.

→ continue reading Every proof assistant: Cur - Designing a less devious proof assistant22 June 2020Andrej Bauer Talks, Every proof assistantWe shall finish the semester with a "Every proof assistant" talk by William Bowman.Note that we start an hour later than usual, at 17:00 UTC+2.

Cur: Designing a less devious proof assistantTime: Thursday, June 25, 2020 from 17:00 to 18:00 (Central European Summer Time, UTC+2)

Location: online at Zoom ID 989 0478 8985

Speaker: William J. Bowman (University of British Columbia)

Proof assistant: CurAbstract:

Dijkstra said that our tools can have a profound and devious influence on our thinking. Ifind this especially true of modern proof assistants, with "devious" out-weighing"profound". Cur is an experiment in design that aims to be less devious. The designemphasizes language extension, syntax manipulation, and DSL construction and integration.This enables the user to be in charge of how they think, rather than requiring the user tocontort their thinking to that of the proof assistant. In this talk, my goal is toconvince you that you want similar capabilities in a proof assistant, and explain anddemonstrate Cur's attempt at solving the problem.

The talk video recording and slides with notes and demo code are available.

Upcoming talks: Anders Mörtberg's talk on Cubical Agda will take place in September 2020.

→ continue reading Every proof assistant: Epigram 2 - Autopsy, Obituary, Apology09 June 2020Andrej Bauer Talks, Every proof assistantThis week shall witness a performance by Conor McBride.

Epigram 2: Autopsy, Obituary, ApologyTime: Thursday, June 11, 2020 from 16:00 to 17:00 (Central European Summer Time, UTC+2)

Location: online at Zoom ID 989 0478 8985

Speaker: Conor McBride (University of Strathclyde)

Proof assistant: Epigram 2Abstract:"A good pilot is one with the same number of take-offs and landings."runs the old joke, which makes me a very bad pilot indeed. The Epigram2 project was repeatedly restarted several times in the late 2000s andnever even reached cruising altitude. This talk is absolutely not anattempt to persuade you to start using it. Rather, it is anexploration of the ideas which drove it: proof irrelevantobservational equality, first class datatype descriptions, nontrivialequational theories for neutral terms. We may yet live to see suchthings. Although the programming language elaborator never happened,the underlying proof engine was accessible via an imperative interfacecalled "Cochon": we did manage some interesting constructions, atleast one of which I can walk through. I'll also explore the reasons,human and technological, why the thing did not survive the long dark.

The video recording of the talk.

Upcoming talks:

June 25, 2020: William J. Bowman, Cur July 2, 2020: Anders Mörtberg - Cubical Agda→ continue reading Every proof assistant: redtt01 June 2020Andrej Bauer Talks, Every proof assistantThis week the speaker will be Jon Sterling, and we are getting two proof assistants for the price of one!

`redtt`

and the future of Cartesian cubical type theoryTime: Thursday, June 4, 2020 from 16:00 to 17:00 (Central European Summer Time, UTC+2)

Location: online at Zoom ID 989 0478 8985

Speaker: Jon Sterling (Carnegie Mellon University)

Proof assistant: redtt and coolttAbstract:

`redtt`

is an interactive proof assistant for Cartesian cubical type theory, a version ofMartin-Löf type theory featuring computational versions of function extensionality, higherinductive types, and univalence. Building on ideas from Epigram, Agda, and Idris,`redtt`

introduces a new cubical take on interactive proof development with holes. We will firstintroduce the basics of cubical type theory and then dive into an interactivedemonstration of`redtt`

’s features and its mathematical library.After this we will catch a first public glimpse of the future of

`redtt`

, a new prototypethat our team is building currently code-named “`cooltt`

”:`cooltt`

introduces syntax tosplit on disjunctions of cofibrations in arbitrary positions, implementing the fulldefinitional eta law for disjunction. While`cooltt`

is still in the early stages, italready has full support for univalence and cubical interactive proof development.The video recording of the talk.

Upcoming talks:

June 11, 2020: Conor McBride - Epigram 2 June 25, 2020: William J. Bowman, Cur July 2, 2020: Anders Mörtberg - Cubical Agda→ continue reading→ later postsPosts→ by category→ by year→ allTAGS:and Mathematics Computation posts Latest

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