On Representation of the Reeb Graph as a Sub-Complex of Manifold

TL;DR

This paper constructs a sub-complex of a manifold representing the Reeb graph of a function, revealing topological properties like the number of loops based on the manifold's fundamental group and genus.

Contribution

It introduces a homotopy equivalent 1-complex embedded in the manifold that models the Reeb graph, providing new insights into its topological structure.

Findings

Reeb graph is a tree for manifolds with finite fundamental group.

Reeb graph contains at most one loop if the fundamental group is abelian or amenable.

Number of loops in Reeb graph on a surface is bounded by the genus.

Abstract

The Reeb graph $\mathcal{R}(f) $ is one of the fundamental invariants of a smooth function $f\colon M\to \mathbb{R} $ with isolated critical points. It is defined as the quotient space $M/_{\!\sim}$ of the closed manifold $M$ by a relation that depends on $f$. Here we construct a $1$-dimensional complex $\Gamma(f)$ embedded into $M$ which is homotopy equivalent to $\mathcal{R}(f)$. As a consequence we show that for every function $f$ on a manifold with finite fundamental group, the Reeb graph of $f$ is a tree. If $\pi_1(M)$ is an abelian group, or more general, a discrete amenable group, then $\mathcal{R}(f)$ contains at most one loop. Finally we prove that the number of loops in the Reeb graph of every function on a surface $M_g$ is estimated from above by $g$, the genus of $M_g$.