Polygonal equalities and $p$-negative type

Abstract

Nontrivial $p$-polygonal equalities impose certain conditions on the geometry of a metric space $(X,d)$ and so it is of interest to be able to identify the values of $p \in [0,\infty)$ for which such equalities exist. Following work of Li and Weston, Kelleher, Miller, Osborn and Weston established that if a metric space $(X,d)$ is of $p$-negative type, then $(X,d)$ admits no nontrivial $p$-polygonal equalities if and only if it is of strict $p$-negative type. In this note we remove the underlying premise of $p$-negative type from this theorem. As an application we show that the set of all $p$ for which a finite metric space $(X,d)$ admits a nontrivial $p$-polygonal equality is always a closed interval of the form $[\wp, \infty)$, where $\wp > 0$, or the empty set. It follows that for each $q \not= 2$, the Schatten $q$-class $\mathcal{C}_{q}$ admits a nontrivial $p$-polygonal equality for each $p > 0$. Other spaces with this same property include $C[0, 1]$ and $\ell_{q}^{(3)}$ for all $q > 2$.