Polynomial braid combing

TL;DR

This paper introduces a polynomial-time algorithm for braid combing, solving the word problem in braid groups on surfaces with boundary, and proposes a simpler algebraic section for surfaces without boundary.

Contribution

It develops a polynomial algorithm for braid combing using straight line programs and introduces an explicit algebraic section for surfaces without boundary.

Findings

Polynomial algorithm for braid combing on surfaces with boundary

First polynomial-time solution for the word problem in these braid groups

Explicit algebraic section for surfaces without boundary

Abstract

Braid combing is a procedure defined by Emil Artin to solve the word problem in braid groups for the first time. It is well-known to have exponential complexity. In this paper, we use the theory of straight line programs to give a polynomial algorithm which performs braid combing. This procedure can be applied to braids on surfaces, providing the first algorithm (to our knowledge) which solves the word problem for braid groups on surfaces with boundary in polynomial time and space. In the case of surfaces without boundary, braid combing needs to use a section from the fundamental group of the surface to the braid group. Such a section was shown to exist by Gon\ccalves and Guaschi, who also gave a geometric description. We propose an algebraically simpler section, which we describe explicitly in terms of generators of the braid group, and we show why the above procedure to comb braids in polynomial time does not work in this case.