1. Department of Mathematics and Burning Plasma Research Group, Politecnico of Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy
2. Department of Mathematics, University of Messina, Salita Sperone 31, 98166 Messina, Italy
3. Department of Energetics and Burning Plasma Research Group, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy