arXiv subjects

Dimitri Ara

Publications and source records attributed to Dimitri Ara.

At least 19 recordsLinked to original sources

arXiv2025

Lax functorialities of the comma construction for $\omega$-categories

Motivated by the Grothendieck construction, we study the functorialities of the comma construction for strict $\omega$-categories. To state the most general functorialities, we use the language of Gray $\omega$-categories, that is, categories enriched in the category of strict $\omega$-categories endowed with the oplax Gray tensor product. Our main result is that the comma construction of strict $\omega$-categories defines a Gray $\omega$-functor, that is, a morphism of Gray $\omega$-categories. To makes sense of this statement, we prove that slices of Gray $\omega$-categories exist. Coming back to the Grothendieck construction, we propose a definition in terms of the comma construction and, as a consequence, we get that the Grothendieck construction of strict $\omega$-categories defines a Gray $\omega$-functor. Finally, as a by-product, we get a notion of Grothendieck construction for Gray $\omega$-functors, which we plan to investigate in future work.

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arXiv2023

Polygraphs: From Rewriting to Higher Categories

Polygraphs are a higher-dimensional generalization of the notion of directed graph. Based on those as unifying concept, this monograph on polygraphs revisits the theory of rewriting in the context of strict higher categories, adopting the abstract point of view offered by homotopical algebra. The first half explores the theory of polygraphs in low dimensions and its applications to the computation of the coherence of algebraic structures. It is meant to be progressive, with little requirements on the background of the reader, apart from basic category theory, and is illustrated with algorithmic computations on algebraic structures. The second half introduces and studies the general notion of n-polygraph, dealing with the homotopy theory of those. It constructs the folk model structure on the category of strict higher categories and exhibits polygraphs as cofibrant objects. This allows extending to higher dimensional structures the coherence results developed in the first half.

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arXiv2022

Orientals as free algebras

The aim of this paper is to give an alternative construction of Street's cosimplicial object of orientals, based on an idea of Burroni that orientals are free algebras for some algebraic structure on strict $\omega$-categories. More precisely, following Burroni, we define the notion of an expansion on an $\omega$-category and we show that the forgetful functor from strict $\omega$-categories endowed with an expansion to strict $\omega$-categories is monadic. By iterating this monad starting from the empty $\omega$-category, we get a cosimplicial object in strict $\omega$-categories. Our main contribution is to show that this cosimplicial object is the cosimplicial objects of orientals. To do so, we prove, using Steiner's theory of augmented directed chain complexes, a general result for comparing polygraphs having same generators and same linearized sources and targets.

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arXiv2022

A categorical characterization of strong Steiner $\omega$-categories

Strong Steiner $\omega$-categories are a class of $\omega$-categories that admit algebraic models in the form of chain complexes, whose formalism allows for several explicit computations. The conditions defining strong Steiner $\omega$-categories are traditionally expressed in terms of the associated chain complex, making them somewhat disconnected from the $\omega$-categorical intuition. The purpose of this paper is to characterize this class as the class of polygraphs that satisfy a loop-freeness condition that does not make explicit use of the associated chain complex and instead relies on the categorical features of $\omega$-categories.

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arXiv2020

Comparison of the $n$-categorical nerves

Our aim is to compare three nerve functors for strict $n$-categories: the Street nerve, the cellular nerve and the multi-simplicial nerve. We show that these three functors are equivalent in some appropriate sense. In particular, the classes of $n$-categorical weak equivalences that they define coincide: they are the Thomason equivalences. We give two applications of this result: the first one states that a Dyer-Kan-type equivalence for Thomason equivalences is a Thomason equivalence; the second one, fundamental, is the stability of the class of Thomason equivalences under the dualities of the category of strict $n$-categories.

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arXiv2019

The folk model category structure on strict $\omega$-categories is monoidal

We prove that the folk model category structure on the category of strict $\omega$-categories, introduced by Lafont, M\'etayer and Worytkiewicz, is monoidal, first, for the Gray tensor product and, second, for the join of $\omega$-categories, introduced by the first author and Maltsiniotis. We moreover show that the Gray tensor product induces, by adjunction, a tensor product of strict $(m,n)$-categories and that this tensor product is also compatible with the folk model category structure. In particular, we get a monoidal model category structure on the category of strict $\omega$-groupoids. We prove that this monoidal model category structure satisfies the monoid axiom, so that the category of Gray monoids, studied by the second author, bears a natural model category structure.

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arXiv2018

A Quillen Theorem B for strict $\infty$-categories

We prove a generalization of Quillen's Theorem B to strict $\infty$-categories. More generally, we show that under similar hypothesis as for Theorem B, the comma construction for strict $\infty$-categories, that we introduced with Maltsiniotis in a previous paper, is the homotopy pullback with respect to Thomason equivalences. We give several applications of these results, including the construction of new models for certain Eilenberg-Mac Lane spaces.

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arXiv2018

A Quillen's Theorem A for strict $\infty$-categories II: the $\infty$-categorical proof

This paper is the second in a series of two papers about generalizing Quillen's Theorem A to strict $\infty$-categories. In the first one, we presented a proof of this Theorem A of a simplicial nature, direct but somewhat ad hoc. In the current paper, we give a conceptual proof of an $\infty$-categorical nature of the same theorem. This proof is based on the theory of join and slices for strict $\infty$-categories developed by the authors in a previous paper, and on a comma construction for strict $\infty$-categories generalizing classical comma categories and Gray's comma 2-categories. This $\infty$-categorical comma construction is used by the first author in another paper to prove a generalization of Quillen's Theorem B to strict $\infty$-categories. We believe that the importance of this comma construction in the theory of $\infty$-categories goes far beyond the scope of homotopy theory.

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arXiv2017

The dendroidal category is a test category

We prove that the category of trees $\Omega$ is a test category in the sense of Grothendieck. This implies that the category of dendroidal sets is endowed with the structure of a model category Quillen-equivalent to spaces. We show that this model category structure, up to a change of cofibrations, can be obtained as an explicit left Bousfield localisation of the operadic model category structure.

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arXiv2017

A Quillen's Theorem A for strict $\infty$-categories I: the simplicial proof

The aim of this paper is to prove a generalization of the famous Theorem A of Quillen for strict $\infty$-categories. This result is central to the homotopy theory of strict $\infty$-categories developed by the authors. The proof presented here is of a simplicial nature and uses Steiner's theory of augmented directed complexes. In a subsequent paper, we will prove the same result by purely $\infty$-categorical methods.

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arXiv2016

Model category structures \`a la Thomason on 2-Cat

In his paper "Th\'eories homotopiques des 2-cat\'egories", Jonathan Chiche studies homotopy theories on 2-Cat, the category of small strict 2-categories, given by classes of weak equivalences which he calls basic localizers of 2-Cat. These basic localizers of 2-Cat are a 2-categorical generalization of the notion of a basic localizer introduced by Grothendieck in "Pursuing stacks". In this paper, we deduce from the results of Jonathan Chiche and results we have obtained with Georges Maltsiniotis that for essentially every basic localizer W of 2-Cat, there exists a model category structure \`a la Thomason on 2-Cat whose weak equivalences are given by W. We show that these model category structures model exactly combinatorial left Bousfield localization of the classical homotopy theory of simplicial sets.

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arXiv2016

Join and slices for strict $\infty$-categories

The goal of this paper is to develop a theory of join and slices for strict $\infty$-categories. To any pair of strict $\infty$-categories, we associate a third one that we call their join. This operation is compatible with the usual join of categories up to truncation. We show that the join defines a monoidal category structure on the category of strict $\infty$-categories and that it respects connected inductive limits in each variable. In particular, we obtain the existence of some right adjoints; these adjoints define $\infty$-categorical slices, in a generalized sense. We state some conjectures about the functoriality of the join and the slices with respect to higher lax and oplax transformations and we prove some first results in this direction. These results are used in another paper to establish a Quillen Theorem A for strict $\infty$-categories. Finally, in an appendix, we revisit the Gray tensor product of strict $\infty$-categories. One of the main tools used in this paper is Steiner's theory of augmented directed complexes.

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arXiv2015

The homotopy type of the $\infty$-category associated to a simplicial complex

This paper is part of a series of papers about homotopy theory of strict $n$-categories. In the first paper of this series, we gave conditions that guarantee the existence of a Thomason model category structure on the category of strict $n$-categories. The main goal of our paper is to show one of these conditions. To do so, we associate to any simplicial complex a strict $\infty$-category generated by a computad. We conjecture that this $\infty$-category has the same homotopy type as the corresponding simplicial complex and we prove this conjecture when the simplicial complex comes from a poset. We introduce the notion of a quasi-initial object of an $\infty$-category and we show that Street's orientals admit such an object. One of the main tools used in this text is Steiner's theory of augmented directed complexes.

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arXiv2013

On autoequivalences of the (\infty, 1)-category of \infty-operads

We study the (\infty, 1)-category of autoequivalences of \infty-operads. Using techniques introduced by To\"en, Lurie, and Barwick and Schommer-Pries, we prove that this (\infty, 1)-category is a contractible \infty-groupoid. Our calculation is based on the model of complete dendroidal Segal spaces introduced by Cisinski and Moerdijk. Similarly, we prove that the (\infty, 1)-category of autoequivalences of non-symmetric \infty-operads is the discrete monoidal category associated to Z/2Z. We also include a computation of the (\infty, 1)-category of autoequivalences of (\infty, n)-categories based on Rezk's \Theta_n-spaces.

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arXiv2013

Towards a Thomason model structure on the category of strict n-categories

The purpose of this article is to present ideas towards obtaining a model category structure on the category of small strict n-categories, generalizing the one obtained by Thomason on ordinary categories. Following ideas of Grothendieck and Cisinski, we obtain an "abstract Thomason theorem", which easily implies the classical Thomason theorem. We deduce a 2-categorical Thomason theorem, an incorrect proof of which has been published by K. Worytkiewicz, K. Hess, P. Parent and A. Tonks. For n > 2, we isolate sufficient conditions to obtain an n-categorical Thomason theorem. These conditions will be investigated in further work.

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arXiv2012

Strict \infty-groupoids are Grothendieck \infty-groupoids

We show that there exists a canonical functor from the category of strict \infty-groupoids to the category of Grothendieck \infty-groupoids and that this functor is fully faithful. As a main ingredient, we prove that free strict \infty-groupoids on a globular pasting scheme are weakly contractible.

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arXiv2012

Higher quasi-categories vs higher Rezk spaces

We introduce a notion of n-quasi-categories as fibrant objects of a model category structure on presheaves on Joyal's n-cell category \Theta_n. Our definition comes from an idea of Cisinski and Joyal. However, we show that this idea has to be slightly modified to get a reasonable notion. We construct two Quillen equivalences between the model category of n-quasi-categories and the model category of Rezk \Theta_n-spaces showing that n-quasi-categories are a model for (\infty, n)-categories. For n = 1, we recover the two Quillen equivalences defined by Joyal and Tierney between quasi-categories and complete Segal spaces.

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arXiv2012

On homotopy types modelized by strict \infty-groupoids

The purpose of this text is the study of the class of homotopy types which are modelized by strict \infty-groupoids. We show that the homotopy category of simply connected \infty-groupoids is equivalent to the derived category in homological degree greater or equal to 2 of abelian groups. We deduce that the simply connected homotopy types modelized by strict \infty-groupoids are precisely the products of Eilenberg-Mac Lane spaces. We also briefly study 3-categories with weak inverses. We finish by two questions about the problem suggested by the title of this text.

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