Self-intersecting curves on a pair of pants and periodic orbits of Hamiltonian flows

Abstract

The character variety $\mathscr{X}(S,G)$ associated to an oriented compact surface $S$ with boundary and a real reductive Lie group $G$ admits a Poisson structure and is foliated by symplectic leaves. When $G$ is a matrix group, any closed curve $c\in\pi_1(S)$ induces a trace function $\mathsf{tr}_c\colon[\rho]\mapsto \mathsf{tr}(\rho(c))$ on $\mathscr{X}(S,G)$. In this article, we study the Hamiltonian flows of trace functions associated to self-intersecting curves. We prove that when $G=\mathsf{PSL}(3,\mathbb{R})$ and $S$ is the pair of pants, every orbit of the Hamiltonian flow of the trace of a figure eight curve on $S$ is periodic and has a unique fixed point. The proof uses explicit computations in Fock-Goncharov coordinates. As an application, we prove a similar statement for the trace of the $\Theta$-web. Finally, we focus on the symplectic leaf corresponding to the unipotent locus, and derive similar results for two more self-intersecting curves: the commutator, and a curve going $k$ times around a boundary component.