1. University Cadi Ayyad, École Nationale des Sciences Appliquées, Safi, Maroc
2. Department of Mathematical Sciences, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy
3. Facultad de Matemática, Astronomía y Física, University of Córdoba, CIEM-CONICET, Córdoba, Argentina