The author thanks Monica Musio for reading previous drafts of the work and Stefano Montaldo for the help in implementing with Mathematica the system of Eqs. (9) in the supplementary material (see Appendix A). This research was partially supported by a grant from the University of Cagliari grant no. F71J12001120002. The author thanks the editor and an anonymous referee for very helpful comments and suggestions. This work was carried out while the author was at Department of Mathematics and Computer Science, University of Cagliari, Italy.