Piecewise circular curves and positivity

Abstract

We introduce the moduli space of generic piecewise circular $n$-gons in the Riemann sphere and relate it to a moduli space of Legendrian polygons. We prove that when $n=2k$, this moduli space contains a connected component homeomorphic to the Fock-Goncharov space of $k$-tuples of positive flags for $\mathsf{PSp}(4,\mathbb{R})$ and hence is a topological ball. We characterize this component geometrically as the space of simple piecewise circular curves with decreasing curvature.