Affiliation: | a Facultatea de Matematica si Informatica, Universitatea din Craiova, Al. I. Cuza 13, Craiova 200585, Romaniab Basque Center for Applied Mathematics, Bizkaia Technology Park, Building 500, E-48160 Derio, Basque Country, Spainc IKERBASQUE - Basque Foundation for Science, E-48011 Bilbao, Basque Country, Spain |