A nonstandard invariant of coarse spaces

Abstract

We construct a set-valued invariant $\iota\left(X,\xi\right)$ of pointed coarse spaces $\left(X,\xi\right)$ by using nonstandard analysis. The invariance under coarse equivalence is established. A sufficient condition for the invariant to be of cardinality $\leq1$ is provided. Miller et al. and subsequent researchers have introduced a similar but standard set-valued coarse invariant $\sigma\left(X,\xi\right)$ of pointed metric spaces $\left(X,\xi\right)$. In order to compare these two invariants, we construct a natural transformation $\omega_{\left(X,\xi\right)}$ from $\sigma\left(X,\xi\right)$ to $\iota\left(X,\xi\right)$. The surjectivity of $\omega_{\left(X,\xi\right)}$ is proved for all proper geodesic spaces $\left(X,\xi\right)$.