  An accidental degeneracy of resonances gives rise to a double pole in the
scattering matrix, a double zero in the Jost function and a Jordan chain of
length two of generalized Gamow-Jordan eigenfunctions of the radial
Schroedinger equation. The generalized Gamow-Jordan eigenfunctions are basis
elements of an expansion in bound and resonant energy eigenfunctions plus a
continuum of scattering wave functions of complex wave number. In this
biorthonormal basis, any operator which is a regular function of the
Hamiltonian is represented by a complex matrix which is diagonal except for a
Jordan block of rank two. The occurrence of a double pole in the Green's
function, as well as the non-exponential time evolution of the Gamow-Jordan
generalized eigenfunctions are associated to the Jordan block in the complex
energy representation.
