# Roe bimodules as morphisms of discrete metric spaces

## TL;DR

This paper explores the category of Roe bimodules as morphisms between discrete metric spaces, revealing how almost isometries influence these morphisms and analyzing their algebraic structures.

## Contribution

It introduces a categorical framework for Roe bimodules between discrete metric spaces and characterizes morphisms via almost isometries, including their algebraic properties.

## Key findings

- Almost isometries determine morphisms in the category
- The set of morphisms on a single space forms a partially ordered semigroup
- Conditions when a morphism forms a $C^*$-algebra

## Abstract

For two discrete metric spaces, $X$ and $Y$ we consider metrics on $X\sqcup Y$ compatible with the metrics on $X$ and $Y$. As morphisms from $X$ to $Y$ we consider the Roe bimodules, i.e. the norm closures of bounded finite propagation operators from $l^2(X)$ to $l^2(Y)$. We study the corresponding category $\mathcal M$, which is also a 2-category. We show that almost isometries determine morphisms in $\mathcal M$. We also consider the case $Y=X$, when there is a richer algebraic structure on the set of morphisms of $\mathcal M$: it is a partially ordered semigroup with the neutral element, with involution, and with a lot of idempotents. We also give a condition when a morphism is a $C^*$-algebra.