Remarks on relativistic Schrodinger operators and their extensions

The operator H = f(-Delta) + V(x) is considered in R-n, where f is a general real increasing function on R+, and V(x) is a real potential with singularities. In particular, the relativistic Schrodinger operator root - Delta + 1 + V(x) is included, Limiting absorption principles are proved for H, including the threshold, The smoothness of the extended resolvents is applied to long-time decay for the evolution group exp(-itH).