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

Abstract

The Reeb graph $\mathcal{R}(f)$ is one of the fundamental invariants of a smooth function $f:M\to \mathbb{R}$ 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 $M_g$.

arXiv comment18 pages