On Burau representations at roots of unity

Abstract

We consider subgroups of the braid groups which are generated by $k$-th powers of the standard generators and prove that any infinite intersection (with even $k$) is trivial. This is motivated by some conjectures of Squier concerning the kernels of Burau's representations of the braid groups at roots of unity. Furthermore, we show that the image of the braid group on 3 strands by these representations is either a finite group, for a few roots of unity, or a finite extension of a triangle group, by using geometric methods.

18p. Former arXiv:0907.0568 is now split as two separate papers: this one records the first half and the title is changed accordingly. The second half is contained in arxiv:1108.4904