Combinatorial and Hodge Laplacians: Similarity and Difference

TL;DR

This paper compares the combinatorial and Hodge Laplacians, introduces Boundary-Induced Graph Laplacians using Discrete Exterior Calculus, and experimentally analyzes their convergence for shape analysis.

Contribution

It introduces BIG Laplacians for boundary-aware discretization and provides a detailed comparison with existing Laplacians, bridging gaps in spectral geometry applications.

Findings

BIG Laplacians characterize data topology and shape.

Eigenvalues of BIG Laplacians converge to Hodge Laplacian eigenvalues for simple shapes.

The paper clarifies the applicability differences between combinatorial and Hodge Laplacians.

Abstract

As key subjects in spectral geometry and combinatorial graph theory respectively, the (continuous) Hodge Laplacian and the combinatorial Laplacian share similarities in revealing the topological dimension and geometric shape of data and in their realization of diffusion and minimization of harmonic measures. It is believed that they also both associate with vector calculus, through the gradient, curl, and divergence, as argued in the popular usage of "Hodge Laplacians on graphs" in the literature. Nevertheless, these Laplacians are intrinsically different in their domains of definitions and applicability to specific data formats, hindering any in-depth comparison of the two approaches. To facilitate the comparison and bridge the gap between the combinatorial Laplacian and Hodge Laplacian for the discretization of continuous manifolds with boundary, we further introduce Boundary-Induced Graph (BIG) Laplacians using tools from Discrete Exterior Calculus (DEC). BIG Laplacians are defined on discrete domains with appropriate boundary conditions to characterize the topology and shape of data. The similarities and differences of the combinatorial Laplacian, BIG Laplacian, and Hodge Laplacian are then examined. Through an Eulerian representation of 3D domains as level-set functions on regular grids, we show experimentally the conditions for the convergence of BIG Laplacian eigenvalues to those of the Hodge Laplacian for elementary shapes.