Antipodal Hadwiger numbers of finite-dimensional Banach spaces

Abstract

Let $X$ be a finite-dimensional Banach space; we introduce and investigate a natural generalization of the concepts of Hadwiger number $H(X)$ and strict Hadwiger number $H'(X)$. More precisely, we define the antipodal Hadwiger number $H_\alpha(X)$ as the largest cardinality of a subset $S \subseteq S_X$, such that $\forall x \neq y \in S \,\,\, \exists f \in B_{X^*}$ with \[1 \le f(x)-f(y) \,\,\, \textrm{and} \,\,\, f(y) \le f(z) \le f(x) \,\,\, \textrm{for} \,\,\, z \in S.\] The strict antipodal Hadwiger number $H'_\alpha(X)$ is defined analogously. We prove that $H'_\alpha(X)=4$ for every Minkowski plane and estimate (or in some cases compute) the numbers $H_\alpha(X)$ and $H'_\alpha(X)$, where $X=\ell_p^n, 1 < p \le +\infty$ and $n \ge 2$. We also show that the number $H'_\alpha(X)$ grows exponentially in $\dim X$.