The limitations of the Poincar{\'e} inequality

Abstract

We examine the validity of the Poincar\'e inequality for degenerate, second-order, elliptic operators $H$ in divergence form on $L_2(\Ri^{n}\times\Ri^{m})$. We assume the coefficients are real symmetric and $a_1H_\delta\geq H\geq a_2H_\delta$ for some $a_1,a_2>0$ where $H_\delta$ is a generalized Gru\v{s}in operator, \[ H_\delta=-\nabla_{x_1}\,|x_1|^{(2\delta_1,2\delta_1')}\,\nabla_{x_1}-|x_1|^{(2\delta_2,2\delta_2')}\,\nabla_{x_2}^2 \;. \] Here $x_1\in\Ri^n$, $x_2\in\Ri^m$, $\delta_1,\delta_1'\in[0,1\rangle$, $\delta_2,\delta_2'\geq0$ and $|x_1|^{(2\delta,2\delta')}=|x_1|^{2\delta}$ if $|x_1|\leq 1$ and $|x_1|^{(2\delta,2\delta')}=|x_1|^{2\delta'}$ if $|x_1|\geq 1$. \smallskip We prove that the Poincar\'e inequality, formulated in terms of the Riemannian geometry corresponding to $H$, is valid if $n\geq 2$, or if $n=1$ and $\delta_1\vee\delta_1'\in[0,1/2\rangle$ but it fails if $n=1$ and $\delta_1\vee\delta_1'\in[1/2,1\rangle$. The failure is caused by the leading term. If $\delta_1\in[1/2, 1\rangle$ it is an effect of the local degeneracy $|x_1|^{2\delta_1}$ but if $\delta_1\in[0, 1/2\rangle$ and $\delta_1'\in [1/2,1\rangle$ it is an effect of the growth at infinity of $|x_1|^{2\delta_1'}$. If $n=1$ and $\delta_1\in[1/2, 1\rangle$ then the semigroup $S$ generated by the Friedrichs' extension of $H$ is not ergodic. The subspaces $x_1\geq 0$ and $x_1\leq 0$ are $S$-invariant and the Poincar\'e inequality is valid on each of these subspaces. If, however, $n=1$, $\delta_1\in[0, 1/2\rangle$ and $\delta_1'\in [1/2,1\rangle$ then the semigroup $S$ is ergodic but the Poincar\'e inequality is only valid locally. \smallskip Finally we discuss the implication of these results for the kernel of the semigroup $S$.