  Let $\mathbf{a}$ be a quadratic form associated with a Schr\"odinger operator
$L=-\nabla\cdot(A\nabla)+V$ on a domain $\Omega\subset \mathbb{R}^d$. If
$\mathbf{a}$ is nonnegative on $C_0^{\infty}(\Omega)$, then either there is
$W>0$ such that $\int W|u|^2 dx\leq \mathbf{a}[u]$ for all
$C_0^{\infty}(\Omega;\mathbb{R})$, or there is a sequence $\phi_k\in
C_0^{\infty}(\Omega)$ and a function $\phi>0$ satisfying $L\phi=0$ such that
$\mathbf{a}[\phi_k]\to 0$, $\phi_k\to\phi$ locally uniformly in
$\Omega\setminus\{x_0\}$. This dichotomy is equivalent to the dichotomy between
$L$ being subcritical resp. critical in $\Omega$. In the latter case, one has
an inequality of Poincar\'e type: there exists $W>0$ such that for every
$\psi\in C_0^\infty(\Omega;\mathbb{R})$ satisfying $\int \psi \phi dx \neq 0$
there exists a constant $C>0$ such that $C^{-1}\int W|u|^2 dx\le
\mathbf{a}[u]+C|\int u \psi dx|^2$ for all $u\in
C_0^\infty(\Omega;\mathbb{R})$.

Analysis of PDEs;Spectral Theory