On the threshold for triangulations inside convex polygons

Abstract

Start with a large convex polygon and add all other edges inside independently with probability $p$. At what critical threshold $p_c$ do triangulations of the polygon begin to appear? The first author and Gravner asked this question, and observed that $p_c=\Theta(1)$, using the relationship with the Catalan numbers and a coupling with oriented site percolation on ${\mathbb Z}^2$. More recently, Archer, Hartarsky, the first author, Olesker-Taylor, Schapira and Valesin proved that $1/4<p_c<p_c^o$, where $1/4$ is the Catalan exponential growth rate and $p_c^o$ is the critical threshold for oriented percolation. The upper bound is strict, but non-quantitative, and follows by a renormalization argument. We show that $p_c<1/2$ using a simple ear clipping algorithm, which can be analyzed using the gambler's ruin problem. This bound is closer to the truth (perhaps near $0.4$) and shows that most configurations of edges inside large convex polygons contain triangulations.