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 establish the latter result, we describe quasi-centralizers in Garside groups. All results hold in the particular case of Artin–Tits groups of spherical type.