1. MIFT, Universita degli Studi di Messina, 18980 Messina, Sicilia, Italy, (e-mail: abuzachis@unime.it)
2. Dept. of Mathematics, University of Messina, Messina, Messina, Italy, (e-mail: acelesti@unime.it)
3. MIFT, Universita degli Studi di Messina, 18980 Messina, Messina, Italy, (e-mail: angalletta@unime.it)
4. South China University of Technology, Guangzhou, Guangdong, China, (e-mail: mejwan@scut.edu.cn)
5. DICIEAMA, University of Messina, Messina, Messina, Italy, (e-mail: mfazio@unime.it)