The eigenvalue problem for the resonances of the infinite-dimensional   Friedrichs model on the positive half line with Hilbert-Schmidt perturbations
  A Gelfand triplet for the Hamiltonian H of the infinite-dimensional Friedrichs model on the positive half line with Hilbert-Schmidt perturbations is constructed such that exactly the resonances (poles of the inverse of the Livsic-matrix) are eigenvalues of the extension H^{\times} of H. The corresponding eigenantilinear forms are calculated explicitly. Using the wave matrices for the Abelian wave (M\"oller) operators the corresponding eigenantilinear forms for the unperturbed Hamiltonian $H_{0}$ turn out to be of pure Dirac type and can be characterized by their corresponding Gamov vector which is uniquely determined by restriction to the intersection of the Gelfand space for $H_{0}$ with $P_{+}H^{2}_{+}$, where $H^{2}_{+}$ is the Hardy space of the upper half plane. Simultaneously, this restriction yields a truncation of the unitary evolution $t\to e^{-itH_{0}}$ to the well-known decay semigroup for $t\geq 0$ of the Toeplitz type on $P_{+}H^{2}_{+}$. That is, exactly those eigenvectors $\lambda\to k(\lambda-\zeta)^{-1}$, $k$ element of the multiplicity space K, of the decay semigroup have an extension to an eigenantilinear form for $H_{0}$ hence for H if $\zeta$ is a resonance and k is from that subspace of K which is uniquely determined by its corresponding Dirac type antilinear form. Moreover, the scattering matrix which is meromorphic in the lower half plane has only simple poles there and the main part of its Laurent representation is a linear combination of Gamov vectors. 
Hellmut Baumg\"artel