1. Department of Mechanics, Mathematics and Management, Polytechnic of Bari, via Orabona 4, Bari 70125, Italy; INFN sez. Lecce, Lecce 73100, Italy e-mail:
2. Professor Department of Mechanics, Mathematics and Management, Polytechnic of Bari, via Orabona 4, Bari 70125, Italy e-mail:
3. Assistant Professor Department of Mechanics, Mathematics and Management, Polytechnic of Bari, via Orabona 4, Bari 70125, Italy; INFN sez. Lecce, Lecce 73100, Italy e-mail: