Approximation of Metric Spaces by Reeb Graphs: Cycle Rank of a Reeb Graph, the Co-rank of the Fundamental Group, and Large Components of Level Sets on Riemannian Manifolds

Author: Irina Gelbukh

Filomat, in print, 2019.

Abstract: For a connected locally path-connected topological space $X$ and a continuous function $f$ on it such that its Reeb graph $R_f$ is a finite topological graph, we show that the cycle rank of $R_f$, i.e., the first Betti number $b_1(R_f)$, in computational geometry called \emph{number of loops}, is bounded from above by the co-rank of the fundamental group $\pi_1(X)$, the condition of local path-connectedness being important since generally $b_1(R_f)$ can even exceed $b_1(X)$. We give some practical methods for calculating the co-rank of $\pi_1(X)$ and a closely related value, the isotropy index. We apply our bound to improve upper bounds on the distortion of the Reeb quotient map, and thus on the Gromov-Hausdorff approximation of the space by Reeb graphs, for the distance function on a compact geodesic space and for a simple Morse function on a closed Riemannian manifold. This distortion is bounded from below by what we call the \emph{Reeb width} $b(M)$ of a metric space $M$, which guarantees that any real-valued continuous function on $M$ has large enough contour (connected component of a level set). We show that for a Riemannian manifold, $b(M)$ is non-zero and give a lower bound on it in terms of characteristics of the manifold. In particular, we show that any real-valued function on a closed Euclidean unit ball $E$ of dimension at least two has a contour $C$ with $\mathrm{diam}(C\cap\partial E)\ge\sqrt{3}$.

Keywords: Reeb graph, co-rank of the fundamental group, metric distortion, Gromov-Hausdorff distance, Riemannian manifolds

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