Spectral positivity and Riemannian coverings
  Let $(M,g)$ be a complete non-compact Riemannian manifold. We consider operators of the form $\Delta_g + V$, where $\Delta_g$ is the non-negative Laplacian associated with the metric $g$, and $V$ a locally integrable function. Let $\rho : (\hat{M},\hat{g}) \to (M,g)$ be a Riemannian covering, with Laplacian $\Delta_{\hat{g}}$ and potential $\hat{V} = V \circ \rho$. If the operator $\Delta + V$ is non-negative on $(M,g)$, then the operator $\Delta_{\hat{g}} + \hat{V}$ is non-negative on $(\hat{M},\hat{g})$. In this note, we show that the converse statement is true provided that $\pi_1(\hat{M})$ is a co-amenable subgroup of $\pi_1(M)$. 
Pierre B\'erard (IF), Philippe Castillon (I3M)
math.DG