A representation of $\text{Out}\left(F_{n}\right)$ by counting subwords of cyclic words

TL;DR

This paper introduces a novel subword counting approach to represent the outer automorphism group of free groups, enabling a sequence of finite matrix approximations of automorphisms.

Contribution

It generalizes previous combinatorial methods to construct a faithful, sequence-based representation of Out(Fn) using subword counts and inverse limits.

Findings

Constructs a finite multiset of words for each automorphism and word

Shows the number of subword occurrences depends only on a specific set of words

Provides a sequence of finite matrices approximating each automorphism

Abstract

We generalize the combinatorial approaches of Rapaport and Higgins--Lyndon to the Whitehead algorithm. We show that for every automorphism $\varphi$ of a free group $F$ and every word $u\in F$ there exists a finite multiset of words $S_{u,\varphi}$ satisfying the following property: For every cyclic word $w$, the number of times $u$ appears as a subword of $\varphi\left(w\right)$ depends only on the appearances of words in $S_{u,\varphi}$ as subwords of $w$. We use this fact to construct a faithful representation of $\text{Out}\left(F_{n}\right)$ on an inverse limit of $\mathbb{Z}$-modules, so that each automorphism is represented by sequence of finite rectangular matrices, which can be seen as successively better approximations of the automorphism.