Affiliation: | a Dipartimento di Matematica e Applicazioni, Università di Napoli-“Federico II”, 80126 Napoli, Italy b Department of Mathematics, Arizona State University, Tempe, AZ 85287, United States c AGH University of Science and Technology, Kraków, Poland d IAC-CNR, Via P. Castellino, 111, 80131 Napoli, Italy |