Interpolation of 2D layered-medium periodic green´s function

In this paper, we investigate various ways to use interpolation to speed up the calculation of the periodic 2D Green´s function in layered media. This corresponds to the calculation of the potential from a 2D infinite periodic set of source points arranged on a lattice in the xy plane, with z being the direction normal to the layers. The lattice vectors sι and s2 in the plane of the sources may be arbitrary, corresponding to a skewed lattice.