# Graph representations of surface flows

## TL;DR

This paper introduces a combinatorial graph-based invariant for classifying non-wandering surface flows with finitely many singular points, enabling an algorithmic enumeration of their topological classes.

## Contribution

It constructs a complete invariant using multi-graphs and saddle connection diagrams, providing a new combinatorial approach to classify such flows.

## Key findings

- Flow classification reduces to combinatorial structures.
- The invariant is complete and can be computed algorithmically.
- Finitely many classes are enumerable, unlike minimal flows.

## Abstract

We construct a complete invariant for non-wandering surface flows with finitely many singular points but without locally dense orbits. Precisely, we show that a flow $v$ with finitely many singular points on a compact connected surface $S$ is a non-wandering flow without locally dense orbits if and only if $S/v_{\mathrm{ex}}$ is a non-trivial embedded multi-graph, where the extended orbit space $S/v_{\mathrm{ex}}$ is the quotient space defined by $x \sim y$ if they belong to either a same orbit or a same multi-saddle connection. Moreover, collapsing edges of the non-trivial embedded multi-graph $S/v_{\mathrm{ex}}$ into singletons, the quotient space $(S/v_{\mathrm{ex}})/\sim_E$ is an abstract multi-graph with the Alexandroff topology with respect to the specialization order. Therefore the non-wandering flow $v$ with finitely many singular points but without locally dense orbits can be reconstruct by finite combinatorial structures, which are the multi-saddle connection diagram and the abstract multi-graph $(S/v_{\mathrm{ex}})/\sim_E$ with labels. Moreover, though the set of topological equivalent classes of irrational rotations (i.e. minimal flows) on a torus is uncountable, the set of topological equivalent classes of non-wandering flows with finitely many singular points but without locally dense orbits on compact surfaces is enumerable by combinatorial structures algorithmically.