  The single-channel Jost function is calculated with the computational
R-matrix on a Lagrange-Jacobi mesh, in order to study its behaviour at complex
wavenumbers. Three potentials derived from supersymmetric transformations are
used to test the accuracy of the method. Each of these potentials, with s-wave
or p-wave bound, resonance or virtual states, has a simple analytical
expression for the Jost function, which is compared with the calculated Jost
function. Siegert states and Siegert pseudostates are determined by finding the
zeros of the calculated Jost function. Poles of the exact Jost function are not
present in the calculated Jost function due to the truncation of the potential
in the R-matrix method. Instead, Siegert pseudostates arise in the vicinity of
the missing poles.
