Triangular-Grid Billiards and Plabic Graphs

Abstract

Given a polygon $P$ in the triangular grid, we obtain a permutation $\pi_P$ via a natural billiards system in which beams of light bounce around inside of $P$. The different cycles in $\pi_P$ correspond to the different trajectories of light beams. We prove that \[\text{area}(P)\geq 6\text{cyc}(P)-6\quad\text{and}\quad\text{perim}(P)\geq\frac{7}{2}\text{cyc}(P)-\frac{3}{2},\] where $\text{area}(P)$ and $\text{perim}(P)$ are the (appropriately normalized) area and perimeter of $P$, respectively, and $\text{cyc}(P)$ is the number of cycles in $\pi_P$. The inequality concerning $\text{area}(P)$ is tight, and we characterize the polygons $P$ satisfying $\text{area}(P)=6\text{cyc}(P)-6$. These results can be reformulated in the language of Postnikov's plabic graphs as follows. Let $G$ be a connected reduced plabic graph with essential dimension $2$. Suppose $G$ has $n$ marked boundary points and $v$ (internal) vertices, and let $c$ be the number of cycles in the trip permutation of $G$. Then we have \[v\geq 6c-6\quad\text{and}\quad n\geq\frac{7}{2}c-\frac{3}{2}.\]