Pr\'esentation des groupes de tresses purs et de certaines de leurs extensions

This paper dates back to 1999 but was never published. The major part of it was included in the joint paper [Digne-Gomi, Presentation of pure braid groups, J. Knot Theory and its Ramifications 10 (2001) 609--623]. Sections 2 and 6 were not included there. They are independent from the rest of the paper. In section 2 we define a morphism from a braid group B associated to a Coxeter Group W to the semi-direct product of ZT (where T is the set of reflections) by W. This morphism lifts the map N from W to (Z/2Z)T defined by Dyer in [Reflection subgroups of Coxeter systems, J. Algebra 135 (1990) 57--73] and its restriction to the pure braid group P identifies with the abelianization morphism. In section 6 we prove that our morphism gives the cocycle associated to the extension B/(P,P) of W by P/(P,P). This extension has been studied recently by Beck [Abelianization of subgroups of reflection groups and their braid groups: an application to cohomology. Manuscripta Math. 136 (2011) 273--293] and by Marin [Reflection groups acting on their hyperplanes J. Algebra 322 (2009) 2848--2860] and [Crystallographic groups and flat manifolds from complex reflection groups, arXiv 1509.06213].