1. Professor Department of Mathematics, Facultad de Ciencias Experimentales, University of Huelva, Avda. Tres de Marzo s/n, 21071 Huelva, Spain e-mail:
2. Professor Department of Applied Mathematics II, E.S. Ingenieros, University of Sevilla, Camino de los Descubrimientos s/n, 41092 Sevilla, Spain e-mail:
3. Professor Department of Applied Mathematics II, E.S. Ingenieros, University of Sevilla, Camino de los Descubrimientos s/n, 41092 Sevilla, Spain e-mail: