A reverse isoperimetric inequality in three-dimensional space forms

Abstract

A $\lambda$-convex body in a three-dimensional space form $M^3(c)$ of constant curvature $c$ is a compact convex set $K$ whose boundary $\partial K$ has normal curvatures bounded below by a constant $\lambda >0$ (in a weak sense). Within this class, we prove a sharp reverse isoperimetric inequality: among all $\lambda$-convex bodies in $M^3(c)$, with a fixed surface area, the body of minimal volume is the $\lambda$-convex lens, i.e., the domain bounded by two totally umbilical caps of curvature $\lambda$. Moreover, this minimizer is unique.