Let $H$ be a Schr\"odinger operator defined on a noncompact Riemannian manifold $\Omega$, and let $W\in L^\infty(\Omega;\mathbb{ R})$. Suppose that the operator $H+W$ is critical in $\Omega$, and let $\varphi$ be the corresponding Agmon ground state. We prove that if $u$ is a generalized eigenfunction of $H$ satisfying $|u|\leq \varphi$ in $\Omega$, then the corresponding eigenvalue is in the spectrum of $H$. The conclusion also holds true if for some $K\Subset \Omega$ the operator $H$ admits a positive solution in $\tilde{ \Omega}=\Omega\setminus K$, and $|u|\leq \psi$ in $\tilde{ \Omega}$, where $\psi$ is a positive solution of minimal growth in a neighborhood of infinity in $\Omega$. Under natural assumptions, this result holds true also in the context of infinite graphs, and Dirichlet forms.
Categories: math.SP math-ph math.AP math.MP