Comparison of spaces of Hardy type for the Ornstein-Uhlenbeck operator

TL;DR

This paper compares local Hardy spaces for the Ornstein-Uhlenbeck operator and explores boundedness properties of imaginary powers, revealing significant differences between these spaces and classical Hardy spaces in various geometric contexts.

Contribution

It introduces a new local Hardy space h^1(g) for the Ornstein-Uhlenbeck operator and compares its properties with existing Hardy spaces, highlighting unboundedness of certain operators.

Findings

Imaginary powers are unbounded from h^1(g) to L^1(g) for all positive r.

Boundedness of operators from H^1(m) to L^1(m) is equivalent to boundedness from h^1(m) to L^1(m) under certain conditions.

Results contrast with classical Euclidean cases and extend to Riemannian manifolds and homogeneous trees.

Abstract

Denote by g the Gauss measure on R^n and by L the Ornstein-Uhlenbeck operator. In this paper we introduce a local Hardy space h^1(g) of Goldberg type and we compare it with the Hardy space H^1(g) introduced in a previous paper by Mauceri and Meda. We show that for each each positive r the imaginary powers of the operator rI+L are unbounded from h^1(g) to L^1(g). This result is in sharp contrast both with the fact that the imaginary powers are bounded from $H^1(g}$ to L^1(g), and with the fact that for the Euclidean laplacian \Delta and the Lebesgue measure \lambda) the imaginary powers of rI-\Delta are bounded from the Goldberg space h^1(\lambda) to L^1(\lambda). We consider also the case of Riemannian manifolds M with Riemannian measure m. We prove that, under certain geometric assumptions on M, an operator T, bounded on L^2(m), and with a kernel satisfying certain analytic assumptions, is bounded from H^1(m) to L^1(m) if and only if it is bounded from h^1(m) to L^1(m). Here H^1(m) denotes the Hardy space on locally doubling metric measure spaces introduced by the authors in arXiv:0808.0146, and h^1(m) is a Goldberg type Hardy space on M, equivalent to a space recently introduced by M. Taylor. The case of translation invariant operators on homogeneous trees is also considered.