An inequality for the compositions of convex functions with convolutions and an alternative proof of the Brunn-Minkowski-Kemperman inequality

Abstract

Let $m(G)$ be the infimum of the volumes of all open subgroups of a unimodular locally compact group $G$. Suppose integrable functions $\phi_1 , \phi_2 \colon G \to [0,1]$ satisfy $\| \phi_1 \| \leq \| \phi_2 \|$ and $\| \phi_1 \| + \| \phi_2 \| \leq m (G)$, where $\| \cdot \|$ denotes the $L^1$-norm with respect to a Haar measure $dg$ on $G$. We have the following inequality for any convex function $f \colon [0, \| \phi_1 \| ] \to \mathbb{R}$ with $f(0) = 0$: \begin{align*} \int_{G}^{} f \circ ( \phi_1 * \phi_2 ) (g) dg \leq 2 \int_{0}^{\| \phi_1 \|} f(y) dy + ( \| \phi_2 \| - \| \phi_1 \| ) f( \| \phi_1 \| ). \end{align*} As a corollary, we have a slightly stronger version of Brunn-Minkowski-Kemperman inequality. That is, we have \begin{align*} \mathrm{vol}_* ( B_1 B_2 ) \geq \mathrm{vol} ( \{ g \in G \mid 1_{B_1} * 1_{B_2} (g) > 0 \} ) \geq \mathrm{vol} (B_1) + \mathrm{vol} (B_2) \end{align*} for any non-null measurable sets $B_1 , B_2 \subset G$ with $\mathrm{vol} (B_1) + \mathrm{vol} (B_2) \leq m(G)$, where $\mathrm{vol}_*$ denotes the inner measure and $1_B$ the characteristic function of $B$.