# The space of invariant measures for countable Markov shifts

## TL;DR

This paper demonstrates that for many non-compact countable Markov shifts, the space of invariant measures forms a Poulsen simplex, extending known results from finite type shifts to more general non-compact settings.

## Contribution

It introduces a topology on measures for non-locally compact spaces and proves the Poulsen simplex structure for invariant measures in broad classes of countable Markov shifts.

## Key findings

- Invariant measure space is a Poulsen simplex minus a vertex.
- The topology of convergence on cylinders generalizes vague topology.
- Results apply to both finite entropy and locally compact shifts.

## Abstract

It is well known that the space of invariant probability measures for transitive sub-shifts of finite type is a Poulsen simplex. In this article we prove that in the non-compact setting, for a large family of transitive countable Markov shifts, the space of invariant sub-probability measures is a Poulsen simplex and that its extreme points are the ergodic invariant probability measures together with the zero measure. In particular we obtain that the space of invariant probability measures is a Poulsen simplex minus a vertex and the corresponding convex combinations. Our results apply to finite entropy non-locally compact transitive countable Markov shifts and to every locally compact transitive countable Markov shift. In order to prove these results we introduce a topology on the space of measures that generalizes the vague topology to a class of non-locally compact spaces, the topology of convergence on cylinders. We also prove analogous results for suspension flows defined over countable Markov shifts.