arXiv subjects

Owen Garnier

Publications and source records attributed to Owen Garnier.

9 recordsLinked to original sources

arXiv2026

Springer categories for regular centralizers in well-generated complex braid groups

In his proof of the K(pi,1) conjecture for complex reflection arrangements, Bessis defined Garside categories suitable for studying braid groups of centralizers of Springer regular elements in well-generated complex reflection groups. We provide a detailed study of these categories, which we call Springer categories. We describe in particular the conjugacy of braided reflections of regular centralizer in terms of the Garside structure of the associated Springer category. In so doing we obtain a pure Garside theoretic proof of a theorem of Digne, Marin and Michel on the center of finite index subgroups in complex braid groups in the case of a regular centralizer in a well-generated group. We also provide a "Hurwitz-like" presentation of Springer categories. To this aim we provide additional insights on noncrossing partitions in the infinite series. Lastly, we use this "Hurwitz-like" presentation, along with a generalized Reidemeister-Schreier method we introduce for groupoids, to deduce nice presentations of the complex braid group B(G31).

math.GR↗

arXiv2026

Proof of Shvartsman's conjecture on braid groups of projective complex reflection groups

The purpose of this note is to prove a conjecture of Shvartsman relating a complex projective reflection group with the quotient of a suitable complex braid group by its center. Shvartsman originally proved this result in the case of real projective reflection groups, and we extend it to all complex projective reflection groups. Our study also allows us to correct a result of Broué, Malle, Rouquier on projective reflection groups.

math.GR↗

arXiv2025

Generalized J-groups, J-braid groups and Seifert link groups

The family of J-groups was introduced by Achar and Aubert with the goal of providing Coxeter-like combinatorial tools for studying rank 2 complex reflection groups. However, J-groups lack an explicit presentation with abstract reflections as generators. This gap was filled by Gobet, and later by the second author, for the subfamily of so-called J-reflection groups. The obtained presentations then gave rise to a concept of J-braid group, which coincides with the link groups of torus necklaces. In this paper we study a generalization of J-groups. We determine which of these groups are finitely generated. We show that, as for classical J-groups, the family of finite generalized J-groups coincides with the family of rank 2 complex reflection groups. We also show that finitely generated generalized J-groups coincide with what we call the torsion quotients of J-braid groups. We deduce explicit presentations for all finitely generated generalized J-groups, where the generators are abstract reflections. We also complete the classification of these groups up to reflection isomorphism. As a byproduct of these results, we obtain that a quotient of a Seifert link group obtained by adding torsion to meridians somehow determines the link up to isotopy. Moreover, such a quotient is finite if and only if it is isomorphic to a complex reflection group of rank two.

math.GR↗

arXiv2025

Conjugacy invariants and rigidity in Garside groups: a uniformity phenomenon

Consider an element~$x$ of a Garside group which is rigid in the sense of Garside-theory. Let $SC(x)$ be the set of rigid conjugates of~$x$ -- this is a well-known characteristic subset of the conjugacy class of~$x$. We present computational evidence that the sequence $( |SC(x^n)| )_{n\in\mathbb N}$ is not only bounded, but in fact periodic, and that there is a bound on the length of the period which depends only on the underlying group and its Garside structure. We prove this result in the special case of the circular Garside groups, including the 2-generator Artin groups with their classical and dual structures (where we prove that the sequence is always constant), and in the case of the dual $4$-strand braid group.

math.GR↗

arXiv2025

Normalisers of parabolic subgroups of Artin--Tits groups and Tits cone intersections

Let $\Gamma$ be a Coxeter diagram and let $J \subseteq \Gamma$. Motivated by 3-fold flops, Iyama and Wemyss study the hyperplane arrangement in the Tits cone intersection of $J$, which is a $J$-relative generalisation of the classical Coxeter arrangement. For $\Gamma$ of finite-type, we show that its complexified hyperplane complement is a $K(\pi,1)$ space for the normaliser (quotient) of the standard parabolic subgroup of the Artin--Tits group attached to $J$. For general $\Gamma$ we show that Brink--Howlett's groupoid, which describes normalisers of parabolic subgroups of Coxeter groups, has its universal cover described by the wall-and-chamber structure of the Tits cone intersection. We use this to show that wall crossing sequences satisfy an "atomic Matsumoto relation", generalising a theorem of Ko and answering questions raised by Iyama and Wemyss.

math.GR↗

arXiv2024

Uniqueness of roots up to conjugacy in circular and hosohedral-type Garside groups

We consider a particular class of Garside groups, which we call circular groups. We mainly prove that roots are unique up to conjugacy in circular groups. This allows us to completely classify these groups up to isomorphism. As a consequence, we obtain the uniqueness of roots up to conjugacy in complex braid groups of rank 2. We also consider a generalization of circular groups, called hosohedral-type groups. These groups are defined using circular groups, and a procedure called the Delta-product, which we study in generality. We also study the uniqueness of roots up to conjugacy in hosohedral-type groups.

math.GR↗

arXiv2024

Parabolic subgroups of complex braid groups: the remaining case

Recently, Marin and González-Meneses introduced a class of ``parabolic'' subgroups for generalized braid groups associated to arbitrary complex reflection groups. Using notably Garside group structures on these generalized braid groups, they proved general results on parabolic subgroups for all cases but one. This last case is that of the complex braid group B(G31), which has no Garside group structure known so far, but instead a Garside groupoid structure. Using this Garside groupoid structure, we complete the results of Marin and González-Meneses by proving their main theorems for the parabolic subgroups of the complex braid group B(G31).

math.GR↗

arXiv2023

Generalization of the Dehornoy-Lafont order complex to categories. Application to exceptional braid groups

The homology of a Garside monoid, thus of a Garside group, can be computed efficiently through the use of the order complex defined by Dehornoy and Lafont. We construct a categorical generalization of this complex and we give some computational techniques which are useful for reducing computing time. We then use this construction to complete results of Salvetti, Callegaro and Marin regarding the homology of exceptional complex braid groups. We most notably study the case of the Borchardt braid group $B(G_{31})$ through its associated Garside category.

math.GR↗

arXiv2023

Regular theory in complex braid groups

In his seminal paper on complex reflection arrangements, Bessis introduces a Garside structure for the braid group of a well-generated irreducible complex reflection group. Using this Garside structure, he establishes a strong connection between regular elements in the reflection group, and roots of the "full twist" element of the pure braid group. He then suggests that it would be possible to extend the conclusion of this theorem to centralizers of regular elements in well-generated groups. In this paper we give a positive answer to this question and we show moreover that these results hold for an arbitrary reflection group.

math.GR↗