  We develop the scattering theory for a pair of self-adjoint operators
$A_{0}=A_{1}\oplus...\oplus A_{N}$ and $A=A_{1}+...+A_{N}$ under the assumption
that all pair products $A_{j}A_{k}$ with $j\neq k$ satisfy certain regularity
conditions. Roughly speaking, these conditions mean that the products
$A_{j}A_{k}$, $j\neq k$, can be represented as integral operators with smooth
kernels in the spectral representation of the operator $A_{0}$. We show that
the absolutely continuous parts of the operators $A_{0}$ and $A$ are unitarily
equivalent. This yields a smooth version of Ismagilov's theorem known earlier
in the trace class framework. We also prove that the singular continuous
spectrum of the operator $A$ is empty and that its eigenvalues may accumulate
only to "thresholds" of the absolutely continuous spectra of the operators
$A_{j}$. Our approach relies on a system of resolvent equations which can be
considered as a generalization of Faddeev's equations for three particle
quantum systems.

Spectral Theory