On Efficient Noncommutative Polynomial Factorization via Higman Linearization

In this paper we study the problem of efficiently factorizing polynomials in the free noncommutative ring F<x_1,x_2,...,x_n> of polynomials in noncommuting variables x_1,x_2,..., x_n over the field F. We obtain the following result: Given a noncommutative arithmetic formula of size s computing a noncommutative polynomial f in F<x_1,x_2,...,x_n> as input, where F=F_q is a finite field, we give a randomized algorithm that runs in time polynomial in s, n and log q that computes a factorization of f as a product f=f_1f_2\cdots f_r, where each f_i is an irreducible polynomial that is output as a noncommutative algebraic branching program. The algorithm works by first transforming f into a linear matrix L using Higman's linearization of polynomials. We then factorize the linear matrix L and recover the factorization of f. We use basic elements from Cohn's theory of free ideals rings combined with Ronyai's randomized polynomial-time algorithm for computing invariant subspaces of a collection of matrices over finite fields.