Changed: more precise statement of the bound on the large x decay of u (Theorem 1.2)
Added: more details in the proof of the large x decay of u (Theorem 1.2)
Fixed: miscellaneous typos.
Added: references to [Nelson, 1964] and [Reed, Simon, 1975].
Fixed: sign of Laplacian in definition of \mathfrak K_e.
Added: references to proofs in introduction.
Fixed: miscellaneous typos.