Hardy and Rellich inequalities with Bessel pairs

Abstract

In this paper, we establish suitable characterisations for a pair of functions $(W(x),H(x))$ on a bounded, connected domain $Ω\subset \mathbb{R}^n$ in order to have the following Hardy inequality \begin{equation*} \int_Ω W(x) |\nabla u|_A^2 dx \geq \int_Ω |\nabla d|^2_AH(x)|u|^2 dx, \,\,\, u \in C^{1}_0(Ω), \end{equation*} where $d(x)$ is a suitable quasi-norm (gauge), $|ξ|^2_A = \langle A(x)ξ, ξ\rangle$ for $ξ\in \mathbb{R}^n$ and $A(x)$ is an $n\times n$ symmetric, uniformly positive definite matrix defined on a bounded domain $Ω\subset \mathbb{R}^n$. We also give its $L^p$ analogue. As a consequence, we present examples for a standard Laplacian on $\mathbb{R}^n$, Baouendi-Grushin operator, and sub-Laplacians on the Heisenberg group, the Engel group and the Cartan group. Those kind of characterisations for a pair of functions $(W(x),H(x))$ are obtained also for the Rellich inequality. These results answer the open problems of Ghoussoub-Moradifam \cite{GM_book}.