Multichannel Green function and perturbation theory for multichannel problems

The Green function of the n-channel Schroedinger equation is expressed in terms of 2n linear-independent solutions of this equation in the general case and in terms of n linear- and channel-independent solutions in the case when the matrix of potentials in Hermitian. The problem of constructing the perturbation series is reduced to a quadrature procedure provided independent solutions of unperturbed equations are known