Christian Sattler

Bio: Christian Sattler is an academic researcher from Chalmers University of Technology. The author has contributed to research in topics: Type theory & Presheaf. The author has an hindex of 6, co-authored 24 publications receiving 143 citations. Previous affiliations of Christian Sattler include University of Nottingham & University of Leeds.

The Equivalence Extension Property and Model Structures

Abstract: We give an elementary construction of a certain class of model structures. In particular, we rederive the Kan model structure on simplicial sets without the use of topological spaces, minimal complexes, or any concrete model of fibrant replacement such as Kan's Ex^infinity functor. Our argument makes crucial use of the glueing construction developed by Cohen et al. in the specific setting of certain cubical sets.

Higher Homotopies in a Hierarchy of Univalent Universes

Abstract: For Martin-Lof type theory with a hierarchy U0:U1:U2:… of univalent universes, we show that Un is not an n-type. Our construction also solves the problem of finding a type that strictly has some high truncation level without using higher inductive types. In particular, Un is such a type if we restrict it to n-types.We have fully formalized and verified our results within the dependently typed language and proof assistant Agda.

Space-Valued Diagrams, Type-Theoretically (Extended Abstract).

Abstract: Topologists are sometimes interested in space-valued diagrams over a given index category, but it is tricky to say what such a diagram even is if we look for a notion that is stable under equivalence. The same happens in (homotopy) type theory, where it is known only for special cases how one can define a type of type-valued diagrams over a given index category. We offer several constructions. We first show how to define homotopy coherent diagrams which come with all higher coherence laws explicitly, with two variants that come with assumption on the index category or on the type theory. Further, we present a construction of diagrams over certain Reedy categories. As an application, we add the degeneracies to the well-known construction of semisimplicial types, yielding a construction of simplicial types up to any given finite level. The current paper is only an extended abstract, and a full version is to follow. In the full paper, we will show that the different notions of diagrams are equivalent to each other and to the known notion of Reedy fibrant diagrams whenever the statement makes sense. In the current paper, we only sketch some core ideas of the proofs.

REVIEWS-Sketches of an elephant: A topos theory compendium

Abstract: A1 Regular and Cartesian Closed Categories A2 Toposes - Basic Theory A3 Allegories A4 Geometric Morphisms - Basic Theory B1 Fibrations and Indexed Categories B2 Internal and Locally Internal Categories B3 Toposes over a base B4 BTop/S as a 2-Category

Cubical agda: a dependently typed programming language with univalence and higher inductive types

Abstract: Proof assistants based on dependent type theory provide expressive languages for both programming and proving within the same system. However, all of the major implementations lack powerful extensionality principles for reasoning about equality, such as function and propositional extensionality. These principles are typically added axiomatically which disrupts the constructive properties of these systems. Cubical type theory provides a solution by giving computational meaning to Homotopy Type Theory and Univalent Foundations, in particular to the univalence axiom and higher inductive types. This paper describes an extension of the dependently typed functional programming language Agda with cubical primitives, making it into a full-blown proof assistant with native support for univalence and a general schema of higher inductive types. These new primitives make function and propositional extensionality as well as quotient types directly definable with computational content. Additionally, thanks also to copatterns, bisimilarity is equivalent to equality for coinductive types. This extends Agda with support for a wide range of extensionality principles, without sacrificing type checking and constructivity.

On Higher Inductive Types in Cubical Type Theory

Abstract: Cubical type theory provides a constructive justification to certain aspects of homotopy type theory such as Voevodsky's univalence axiom. This makes many extensionality principles, like function and propositional extensionality, directly provable in the theory. This paper describes a constructive semantics, expressed in a presheaf topos with suitable structure inspired by cubical sets, of some higher inductive types. It also extends cubical type theory by a syntax for the higher inductive types of spheres, torus, suspensions, truncations, and pushouts. All of these types are justified by the semantics and have judgmental computation rules for all constructors, including the higher dimensional ones, and the universes are closed under these type formers.