Mixed-Parabolicity and Mixed-Liouville Property for Products of Riemannian Manifolds

Abstract

Let $p_1,p_2\in (1,\infty )$ and $M=M_1\times M_2$ be the product of two geodesically complete Riemannian manifolds. In this paper, the authors first develop an anisotropic potential-theoretic framework adapted to the Green operator $G^M$ and the mixed-norm Lebesgue space $L^{p_2}(L^{p_1})(M)$, and then demonstrate that the classical equivalence among \emph{parabolicity}, \emph{Green function integrability}, and \emph{Liouville property} persists in this genuinely anisotropic setting. More precisely, the authors establish the following equivalence: $M$ is $L^{p_2}(L^{p_1})$-parabolic if and only if the Green function $G^M(x;\cdot )$ fails to belong to $L^{p_2^{\prime }}(L^{p_1^{\prime }})(M\setminus B(x,r))$, which is in turn equivalent to the $L^{p_2^{\prime }}(L^{p_1^{\prime }})$-Liouville property, where $p_i^{\prime }$ denotes the conjugate exponent of $p_i$. Under a weak radial Harnack-type inequality -- in particular, under Li--Yau heat kernel estimates, and hence for products of manifolds with nonnegative Ricci curvature -- these conditions are further equivalent to the divergence of the nonlinear mixed-potential ${\mathcal{G}}_{p_1,p_2}(f)$ for every nonzero nonnegative $f\in {\mathcal{C}}_c^{\infty }(M)$. A key feature of this anisotropic theory is its sensitivity to the geometry of each factor \(M_i\), rather than merely to that of the total manifold \(M\). In contrast to the isotropic case, where parabolicity and the classical Liouville property holds on \(\mathbb{R}^n\) precisely when \(n \le 2\), the anisotropic setting exhibits a refined threshold: the \(L^{p_2}(L^{p_1})\)-parabolicity and the \(L^{p_2'}(L^{p_1'})\)-Liouville property holds on \(\mathbb{R}^{n_1} \times \mathbb{R}^{n_2}\) if and only if $D_{\mathrm{eff}}:=\frac{n_1}{p_1}+\frac{n_2}{p_2}\le 2.$ This effective dimension $D_{\mathrm{eff}}$ captures the anisotropic interplay between the exponents \(p_1, p_2\) and the geometries of \(M_1, M_2\).

50 pages