arXiv2010
The Brunn-Minkowski Theorem asserts that $μ_d(A+B)^{1/d}\geq μ_d(A)^{1/d}+μ_d(B)^{1/d}$ for convex bodies $A,\,B\subseteq \R^d$, where $μ_d$ denotes the $d$-dimensional Lebesgue measure. It is well-known that equality holds if and only if $A$ and $B$ are homothetic, but few characterizations of equality in other related bounds are known. Let $H$ be a hyperplane. Bonnesen later strengthened this bound by showing $$μ_d(A+B)\geq (M^{1/(d-1)}+N^{1/(d-1)})^{d-1}(\frac{μ_d(A)}{M}+\frac{μ_d(B)}{N}),$$ where $M=\sup\{μ_{d-1}((\mathbf x+H)\cap A)\mid \mathbf x\in \R^d\}$ and $N=\sup\{μ_{d-1}((\mathbf y+H)\cap B)\mid \mathbf y\in \R^d\}$. Standard compression arguments show that the above bound also holds when $M=μ_{d-1}(π(A))$ and $N=μ_{d-1}(π(B))$, where $π$ denotes a projection of $\mathbb R^d$ onto $H$, which gives an alternative generalization of the Brunn-Minkowski bound. In this paper, we characterize the cases of equality in this later bound, showing that equality holds if and only if $A$ and $B$ are obtained from a pair of homothetic convex bodies by `stretching' along the direction of the projection, which is made formal in the paper. When $d=2$, we characterize the case of equality in the former bound as well.