Minmax bornologies

Abstract

A bornology $\mathcal{B}$ on a set $X$ is called minmax, if the smallest and largest coarse structures on $X$ compatible with $\mathcal{B}$ coincide. We prove that $\mathcal{B}$ is minmax, if and only if the family $\mathcal B^\sharp=\{p\in\beta X:\{X\setminus B:B\in\mathcal B\}\subset p\}$ consists of ultrafilters which are pairwise non-isomorphic via $\mathcal B$-preserving bijections of $X$. In addition, we construct a minmax bornology $\mathcal B$ on $\omega$ such that the set $\mathcal B^\sharp$ is infinite. We deduce this result from the existence of a closed infinite subset in $\beta\omega$ that consists of pairwise non-isomorphic ultrafilters.