Spectral triples, Coulhon-Varopoulos dimension and heat kernel estimates

TL;DR

This paper explores the relationship between Coulhon-Varopoulos and spectral dimensions in various geometric and noncommutative settings, revealing conditions under which they coincide or diverge, and implications for von Neumann algebras.

Contribution

It establishes that the Coulhon-Varopoulos dimension bounds the spectral dimension and shows their potential divergence in non-smooth settings, also linking semigroup properties to algebraic injectivity.

Findings

Coulhon-Varopoulos dimension exceeds spectral dimension in general.

In smooth compact settings, the dimensions often coincide.

Finite Coulhon-Varopoulos dimension implies von Neumann algebra injectivity.

Abstract

We investigate the relations between the (completely bounded) local Coulhon-Varopoulos dimension and the spectral dimension of spectral triples associated to sub-Markovian semigroups (or Dirichlet forms) acting on classical (or noncommutative) $\mathrm{L}^p$-spaces associated to finite measure spaces. More precisely, we prove that the completely bounded local Coulhon-Varopoulos dimension $d$ exceeds the spectral dimension and even implies that the associated Hodge-Dirac operator is $d^+$-summable. We explore different settings to compare these two values: compact Riemannian manifolds, compact Lie groups, sublaplacians, metric measure spaces, noncommutative tori and quantum groups. Specifically, we prove that, while very often equal in smooth compact settings, these dimensions can diverge. Finally, we show that the existence of a symmetric sub-Markovian semigroup on a von Neumann algebra with finite completely bounded local Coulhon-Varopoulos dimension implies that the von Neumann algebra is necessarily injective.