  We prove that each Borel function $V : \Omega \to [-\infty, +\infty]$ defined
on an open subset $\Omega \subset \mathbb{R}^{N}$ induces a decomposition
$\Omega = S \cup \bigcup_{i} D_{i}$ such that every function in
$W^{1,2}_{0}(\Omega) \cap L^{2}(\Omega; V^{+} dx)$ is zero almost everywhere on
$S$ and existence of nonnegative supersolutions of $-\Delta + V$ on each
component $D_{i}$ yields nonnegativity of the associated quadratic form
$\int_{D_{i}} (|\nabla \xi|^2+V\xi^2)$.
