Heat kernel estimates and the relative compactness of perturbations by   potentials
  We consider a self-adjoint non-negative operator $H$ in a Hilbert space $\mathsf{L}^2(X,{\rm d}\mu)$. We assume that the semigroup $(\mathrm{e}^{-t H})_{t>0}$ is defined by an integral kernel, $p$, which allows an estimate of the form $p(t,x,x)\le F_1(x)F_2(t)$ for all $(x,t)\in X\times\mathbb{R_+}$; we refer to $F_1$ as the \emph{control function}. We show that such an estimate leads to rather satisfying abstract results on relative compactness of perturbations of $H$ by potentials. It came as a surprise to us, however, that such an estimate holds for the Laplace-Beltrami operator on \emph{any} Riemannian manifold. In particular, using a domination principle, one can deduce from the latter fact a very general result on the relative compactness of perturbations by potentials of the Bochner Laplacian associated with a Hermitian bundle $(E, h^E,\nabla^E)$ over an arbitrary Riemannian manifold $(M,g)$; in fact, only quantities of order zero in $g$ enter in the estimates. We extend this result to weighted Riemannian manifolds, where under lower curvature bounds on the $\alpha$-Bakry-\'{E}mery tensor one can construct quite explicit control functions, and to any weighted graph, where the control function is expressed in terms of the vertex weight function. 
Jochen Br\"uning, Batu G\"uneysu
math.SP math.DG math.FA