Some functional properties on Cartan-Hadamard manifolds of very negative curvature

Abstract

In this paper we consider Cartan-Hadamard manifolds (i.e. simply connected of non-positive sectional curvature) whose negative Ricci curvature grows polynomially at infinity. We show that a number of functional properties, which typically hold when the curvature is bounded, remain true in this setting. These include the characterization of Sobolev spaces on manifolds, the so-called Cald\'eron-Zygmund inequalities and the $L^p$-positivity preserving property, i.e. $u\in L^p\ \&\ (-\Delta + 1)u\ge 0 \Rightarrow u\ge 0$. The main tool is a new class of first and second order Hardy-type inequalities on Cartan-Hadamard manifolds with a polynomial upper bound on the curvature. In the last part of the manuscript we prove the $L^p$-positivity preserving property, $p\in[1,+\infty]$, on manifolds with subquadratic negative part of the Ricci curvature. This generalizes an idea of B. G\"uneysu and gives a new proof of a well-known condition for the stochastic completeness due to P. Hsu.