# Max-plus convexity in Riesz spaces

## TL;DR

This paper explores max-plus convexity within Riesz spaces, establishing algebraic definitions, associated norms, and geodesic distances, and extends fixed point and selection theorems to this setting, showing hyperspaces are Absolute Retracts.

## Contribution

It introduces a new algebraic framework for max-plus convexity in Riesz spaces and extends classical convexity results to this non-linear context without assuming topological structures.

## Key findings

- Max-plus convex sets are geodesically closed under certain norms.
- Fixed point and continuous selection theorems are extended to max-plus convex sets.
- Hyperspaces of compact max-plus convex sets are Absolute Retracts.

## Abstract

We study max-plus convexity in an Archimedean Riesz space $E$ with an order unit $\un$; the definition of max-plus convex sets is algebraic and we do not assume that $E$ has an {\it a priori} given topological structure. To the given unit $\un$ one can associate two equivalent norms $\norm\cdot\norm_{\un}$ and $\norm\cdot\norm_{\hun}$ on $E$; the distance ${\sf D}_{\hun}$ on $E$ associated to $\norm\cdot\norm_{\hun}$ is a geodesic distance for which max-plus convex sets in $E$ are geodesically closed sets. Under suitable assumptions, we establish max-plus versions of some fixed points and continuous selection theorems that are well known for linear convex sets and we show that hyperspaces of compact max-plus convex sets are Absolute Retracts.