2015/11/27 by François Digne, Digne, François
Mathematics · #FOS: Mathematics #Group Theory (math.GR) #math.GR
paper · pdf · doi:10.48550/arxiv.1511.08731
in French. This version: a correction in definition 2.4
arxiv created 2016/01/07 · arxiv updated 2016/01/08
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].