V.Hardt, R. Mennicken, A. K. Motovilov
Factorization Theorem for the Transfer Function 
of a $2\times2$ Operator Matrix with Unbounded Couplings
(168K, PostScript)

ABSTRACT.  We consider the analytic continuation of the transfer function 
associated with a $2\times2$ operator matrix having unbounded 
couplings into unphysical sheets of its Riemann surface. We 
construct a family of non-selfadjoint operators which factorize 
the transfer function and reproduce certain parts of its 
spectrum including the nonreal (resonance) spectrum situated in 
the unphysical sheets neighboring the physical sheet.