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David Bessis

Publications and source records attributed to David Bessis.

13 recordsLinked to original sources
arXiv2014

Finite complex reflection arrangements are K(pi,1)

Let $V$ be a finite dimensional complex vector space and $W\subseteq \GL(V)$ be a finite complex reflection group. Let $V^{\reg}$ be the complement in $V$ of the reflecting hyperplanes. We prove that $V^{\reg}$ is a $K(π,1)$ space. This was predicted by a classical conjecture, originally stated by Brieskorn for complexified real reflection groups. The complexified real case follows from a theorem of Deligne and, after contributions by Nakamura and Orlik-Solomon, only six exceptional cases remained open. In addition to solving this six cases, our approach is applicable to most previously known cases, including complexified real groups for which we obtain a new proof, based on new geometric objects. We also address a number of questions about $π_1(W\cq V^{\reg})$, the braid group of $W$. This includes a description of periodic elements in terms of a braid analog of Springer's theory of regular elements.

math.GT↗
arXiv2006

Garside categories, periodic loops and cyclic sets

Garside groupoids, as recently introduced by Krammer, generalise Garside groups. A weak Garside group is a group that is equivalent as a category to a Garside groupoid. We show that any periodic loop in a Garside groupoid $\CG$ may be viewed as a Garside element for a certain Garside structure on another Garside groupoid $\CG_m$, which is equivalent as a category to $\CG$. As a consequence, the centraliser of a periodic element in a weak Garside group is a weak Garside group. Our main tool is the notion of divided Garside categories, an analog for Garside categories of Bökstedt-Hsiang-Madsen's subdivisions of Connes' cyclic category. This tool is used in our separate proof of the $K(π,1)$ property for complex reflection arrangements

math.GR↗
arXiv2004

Topology of complex reflection arrangements

Let $V$ be a finite dimensional complex vector space and $W\subset \GL(V)$ be a finite complex reflection group. Let $V^{\reg}$ be the complement in $V$ of the reflecting hyperplanes. A classical conjecture predicts that $V^{\reg}$ is a $K(pi,1)$ space. When $W$ is a complexified real reflection group, the conjecture follows from a theorem of Deligne. Our main result validates the conjecture for duality (or, equivalently, well-generated) complex reflection groups. This includes the complexified real case (but our proof is new) and new cases not previously known. We also address a number of questions about $π_1(W\cq V^{\reg})$, the braid group of $W$.

math.GT↗
arXiv2004

Non-crossing partitions of type (e,e,r)

We investigate a new lattice of generalised non-crossing partitions, constructed using the geometry of the complex reflection group $G(e,e,r)$. For the particular case $e=2$ (resp. $r=2$), our lattice coincides with the lattice of simple elements for the type $D_n$ (resp. $I_2(e)$) dual braid monoid. Using this lattice, we construct a Garside structure for the braid group $B(e,e,r)$. As a corollary, one may solve the word and conjugacy problems in this group.

math.GR↗
arXiv2004

A dual braid monoid for the free group

We construct a quasi-Garside monoid structure for the free group. This monoid should be thought of as a dual braid monoid for the free group, generalising the constructions by Birman-Ko-Lee and by the author of new Garside monoids for Artin groups of spherical type. Conjecturally, an analog construction should be available for arbitrary Artin groups and for braid groups of well-generated complex reflection groups.

math.GR↗
arXiv2003

Explicit presentations for exceptional braid groups

We give presentations for the braid groups associated with the complex reflection groups $G_{24}$ and $G_{27}$. For the cases of $G_{29}$, $G_{31}$, $G_{33}$ and $G_{34}$, we give (strongly supported) conjectures. These presentations were obtained with VKCURVE, a GAP package implementing Van Kampen's method.

math.GR↗
arXiv2003

Garside structure for the braid group of G(e,e,r)

We give a new presentation of the braid group $B$ of the complex reflection group $G(e,e,r)$ which is positive and homogeneous, and for which the generators map to reflections in the corresponding complex reflection group. We show that this presentation gives rise to a Garside structure for $B$ with Garside element a kind of generalised Coxeter element, and hence obtain solutions to the word and conjugacy problems for $B$.

math.GR↗
arXiv2003

Variations on Van Kampen's method

We give a detailed account of the classical Van Kampen method for computing presentations of fundamental groups of complements of complex algebraic curves, and of a variant of this method, working with arbitrary projections (even with vertical asymptotes).

math.GR↗
arXiv2002

The dual braid monoid

We construct a new monoid structure for Artin groups associated with finite Coxeter systems. This monoid shares with the classical positive braid monoid a crucial algebraic property: it is a Garside monoid. The analogy with the classical construction indicates there is a ``dual'' way of studying Coxeter systems, where the pair (W,S) is replaced by (W,T), with T the set of all reflections. In the type A case, we recover the monoid constructed by Birman-Ko-Lee

math.GR↗
arXiv2001

Quotients et extensions de groupes de reflexions complexes

We give a geometric description of a certain class of epimorphisms between complex reflection groups. We classify these epimorphisms, which can be interpreted as ``morphisms'' between the diagrams symbolizing standard presentations by generators and relations for complex reflection groups and their braid groups.

math.GR↗
arXiv2001

Zariski theorems and diagrams for braid groups

Empirical properties of generating systems for complex reflection groups and their braid groups have been observed by Orlik-Solomon and Broué-Malle-Rouquier, using Shephard-Todd classification. We give a general existence result for presentations of braid groups, which partially explains and generalizes the known empirical properties. Our approach is invariant-theoretic and does not use the classification. The two ingredients are Springer theory of regular elements and a Zariski-like theorem.

math.GR↗
arXiv2000

Springer theory in braid groups and the Birman-Ko-Lee monoid

We state a conjecture about centralizers of certain roots of central elements in braid groups, and check it for Artin braid groups and some other cases. Our proof makes use of results by Birman-Ko-Lee; we give a new intrinsic account of these results.

math.GR↗