1. Department of Mathematics and Computer Science, University of Perugia, 06123 Perugia, Italy
2. École Supérieure de Technologie Essaouira, Cadi Ayyad University, Essaouira 44000, Morocco
3. Departamento de Matemática Aplicada, University of Granada, 18071 Granada, Spain
4. Department of Mathematics, Politecnico Torino, 10129 Turin, Italy