Palindromic Automorphisms of Free Groups

Let $F_n$ be the free group of rank $n$ with free basis $X=\{x_1,\dots,x_n \}$. A palindrome is a word in $X^{\pm 1}$ that reads the same backwards as forwards. The palindromic automorphism group $\Pi A_n$ of $F_n$ consists of those automorphisms that map each $x_i$ to a palindrome. In this paper, we investigate linear representations of $\Pi A_n$, and prove that $\Pi A_2$ is linear. We obtain conjugacy classes of involutions in $\Pi A_2$, and investigate residual nilpotency of $\Pi A_n$ and some of its subgroups. Let $IA_n$ be the group of those automorphisms of $F_n$ that act trivially on the abelianisation, $P I_n$ be the palindromic Torelli group of $F_n$, and $E \Pi A_n$ be the elementary palindromic automorphism group of $F_n$. We prove that $PI_n=IA_n \cap E \Pi A_n'$. This result strengthens a recent result of Fullarton.