A quantitative Robin P\'olya--Szeg\H{o} theorem for tangential polygons

Abstract

We extend the quantitative Dirichlet theorem of Gui, Hu, and Li \cite{GuiHuLi2026} to Robin torsional rigidity. For every fixed $\beta>0$, we prove that the regular $N$-gon uniquely maximizes Robin torsional rigidity among tangential $N$-gons of prescribed area. Our explicit perimeter deficit bound contains the Dirichlet deficit obtained by Gui, Hu, and Li \cite{GuiHuLi2026} and an additional Robin term that is positive for nonregular tangential polygons. For triangles, we answer the fixed-area optimization problem recorded as open by Keady and Wiwatanapataphee \cite[Appendix A.1.2]{KeadyWiwatanapataphee2016}. We also establish strict monotonicity along equal-area regular polygons as the number of sides increases, with an explicit bound for each consecutive gap. The proof combines mixed Robin--Neumann triangular cells, spectral estimates for angular concavity, and a quantitative scaling estimate.