Parabolic subgroups of Garside groups II: ribbons

We introduce and investigate the ribbon groupoid associated with a Garside group. Under a technical hypothesis, we prove that this category is a Garside groupoid. We decompose this groupoid into a semi-direct product of two of its parabolic subgroupoids and provide a groupoid presentation. In order to established the latter result, we describe quasi-centralizers in Garside groups. All results hold in the particular case of Artin-Tits groups of spherical type

Generic Hecke algebra for Renner monoids

We associate with every Renner monoid $R$ a \emph{generic Hecke algebra} $\H(R)$ over $\mathbb{Z}[q]$ which is a deformation of the monoid $\mathbb{Z}$-algebra of $R$. If $M$ is a finite reductive monoid with Borel subgroup $B$ and associated Renner monoid $R$, then we obtain the associated Iwahori-Hecke algebra $\H(M,B)$ by specialising $q$ in $\H(R)$ and tensoring by $\mathbb{C}$ over $\mathbb{Z}$, as in the classical case of finite algebraic groups. This answers positively to a long-standing question of L. Solomon

Basic Questions on Artin-Tits groups

This paper is a short survey on four basic questions on Artin-Tits groups: the torsion, the center, the word problem, and the cohomology ($K(\pi,1)$ problem). It is also an opportunity to prove three new results concerning these questions: (1) if all free of infinity Artin-Tits groups are torsion free, then all Artin-Tits groups will be torsion free; (2) If all free of infinity irreducible non-spherical type Artin-Tits groups have a trivial center then all irreducible non-spherical type Artin-Tits groups will have a trivial center; (3) if all free of infinity Artin-Tits groups have solutions to the word problem, then all Artin-Tits groups will have solutions to the word problem. Recall that an Artin-Tits group is free of infinity if its Coxeter graph has no edge labeled by $\infty$

A conjecture about Artin-Tits groups

We conjecture that the word problem of Artin-Tits groups can be solved without introducing trivial factors ss^{-1} or s^{-1}s. Here we make this statement precise and explain how it can be seen as a weak form of hyperbolicity. We prove the conjecture in the case of Artin-Tits groups of type FC, and we discuss various possible approaches for further extensions, in particular a syntactic argument that works at least in the right-angled case

Questions on surface braid groups

We provide new group presentations for surface braid groups which are positive. We study some properties of such presentations and we solve the conjugacy problem in a particular case

Folding of set-theoretical solutions of the Yang-Baxter equation

We establish a correspondence between the invariant subsets of a non-degenerate symmetric set-theoretical solution of the quantum Yang-Baxter equation and the parabolic subgroups of its structure group, equipped with its canonical Garside structure. Moreover, we introduce the notion of a foldable solution, which extends the one of a decomposable solution

K(π,1)K(\pi,1) and word problems for infinite type Artin-Tits groups, and applications to virtual braid groups

Let $\Gamma$ be a Coxeter graph, let $(W,S)$ be its associated Coxeter system, and let $(A,\Sigma$) be its associated Artin-Tits system. We regard $W$ as a reflection group acting on a real vector space $V$. Let $I$ be the Tits cone, and let $E_\Gamma$ be the complement in $I +iV$ of the reflecting hyperplanes. Recall that Charney, Davis, and Salvetti have constructed a simplicial complex $\Omega(\Gamma)$ having the same homotopy type as $E_\Gamma$. We observe that, if $T \subset S$, then $\Omega(\Gamma_T)$ naturally embeds into $\Omega (\Gamma)$. We prove that this embedding admits a retraction $\pi_T: \Omega(\Gamma) \to \Omega (\Gamma_T)$, and we deduce several topological and combinatorial results on parabolic subgroups of $A$. From a family \SS of subsets of $S$ having certain properties, we construct a cube complex $\Phi$, we show that $\Phi$ has the same homotopy type as the universal cover of $E_\Gamma$, and we prove that $\Phi$ is CAT(0) if and only if \SS is a flag complex. We say that $X \subset S$ is free of infinity if $\Gamma_X$ has no edge labeled by $\infty$. We show that, if $E_{\Gamma_X}$ is aspherical and $A_X$ has a solution to the word problem for all $X \subset S$ free of infinity, then $E_\Gamma$ is aspherical and $A$ has a solution to the word problem. We apply these results to the virtual braid group $VB_n$. In particular, we give a solution to the word problem in $VB_n$, and we prove that the virtual cohomological dimension of $VB_n$ is $n-1$