On $k$-convex hulls

Davide Ravasini

Published 2024-11-21Version 1

For every integer $k\geq 2$ and every $R>1$ one can find a dimension $n$ and construct a symmetric convex body $K\subset\mathbb{R}^n$ with $\text{diam} Q_{k-1}(K)\geq R\cdot\text{diam} Q_k(K)$, where $Q_k(K)$ denotes the $k$-convex hull of $K$. The purpose of this short note is to show that this result due to E. Kopeck\'{a} is impossible to obtain if one additionally requires that all isometric images of $K$ satisfy the same inequality. To this end, we introduce the dual construction to the $k$-convex hull of $K$, which we call $k$-cross approximation.