Spectral theory of Schr\"odinger operators with infinitely many point   interactions and radial positive definite functions
  A number of results on radial positive definite functions on ${\mathbb R^n}$ related to Schoenberg's integral representation theorem are obtained. They are applied to the study of spectral properties of self-adjoint realizations of two- and three-dimensional Schr\"odinger operators with countably many point interactions. In particular, we find conditions on the configuration of point interactions such that any self-adjoint realization has purely absolutely continuous non-negative spectrum. We also apply some results on Schr\"odinger operators to obtain new results on completely monotone functions. 
Mark M. Malamud and Konrad Schm\"udgen
math.FA