The past and future wave operators on the singular spectrum
  We consider averaged wave operators constructed for singular unitary operators $U_1$, $U_2$ and a bounded identification operator $A$. In the case of rank-two commutator $AU_1 - U_2A$, we show that averaged wave operators of past and future exist or do not exist simultaneously, and if they exist, they must coincide. As a consequence, we obtain some results concerning boundary behaviour of Cauchy-type integrals. 
R. V. Bessonov
math.FA math.CV