# Quantum confinement on non-complete Riemannian manifolds

### Dario Prandi, Luca Rizzi and Marcello Seri

**Abstract:**
We consider the quantum completeness problem, i.e.\ the problem of confining
quantum particles, on a non-complete Riemannian manifold $M$ equipped with a
smooth measure $\omega$, possibly degenerate or singular near the metric
boundary of $M$, and in presence of a real-valued potential $V\in L^2_ \mathrm{loc}(M)$.
The main merit of this paper is the identification of an intrinsic quantity, the
effective potential $V_{\mathrm{Eff}}$, which allows to formulate simple criteria for
quantum confinement. Let $\delta$ be the distance from the possibly non-compact
metric boundary of $M$. A simplified version of the main result guarantees
quantum completeness if $V\ge -c\delta^2$ far from the metric boundary and

These criteria allow us to: (i) obtain quantum confinement results for measures with degeneracies or singularities near the metric boundary of $M$; (ii) generalize the Kalf-Walter-Schmincke-Simon Theorem for strongly singular potentials to the Riemannian setting for any dimension of the singularity; (iii) give the first, to our knowledge, curvature-based criteria for self-adjointness of the Laplace-Beltrami operator; (iv) prove, under mild regularity assumptions, that the Laplace-Beltrami operator in almost-Riemannian geometry is essentially self-adjoint, partially settling a conjecture formulated in BL.