Mathematical Model of Resonances and Tunneling in a System with a Bound State

We study the asymptotic behavior of the residue at the pole of the analytic continuation of the scattering matrix as the imaginary part of the pole tends to zero in the case where the phase space of a quantum mechanical system is a direct sum of two spaces and the nonperturbed evolution operator reduces each of these spaces and has a discrete spectrum in one of them and a continuous spectrum in the other. The perturbation operator mixes the subspaces and generates a resonance. We prove that under certain symmetry conditions in such a system, the scattering amplitude changes sharply in a neighborhood of the real part of the pole of the scattering matrix, and the system demonstrates tunneling or a resonance of the scattering amplitude.