On Cayley Identity for Self-Adjoint Operators in Hilbert Spaces
  We prove an analogue to the Cayley identity for an arbitrary self-adjoint operator in a Hilbert space. We also provide two new ways to characterize vectors belonging to the singular spectral subspace in terms of the analytic properties of the resolvent of the operator, computed on these vectors. The latter are analogous to those used routinely in the scattering theory for the absolutely continuous subspace. 
Alexander V. Kiselev and Serguei Naboko
math.SP