  In this paper we prove for rank one perturbations that negative two times
reciprocal of the imaginary part of resonance point is equal to the rate of
change of the scattering phase as a function of the coupling constant, where
the coupling constant is equal to the real part of the resonance point. This
equality is in agreement with Breit-Wigner formula from quantum scattering
theory.
  For general relatively trace class perturbations, we also give a formula for
the spectral shift function in terms of resonance points, non-real and real.
