[1] Univ Salerno, Dipartimento Matemat & Informat, I-84081 Baronissi, Italy
[2] INFM, Unita Salerno, I-84081 Baronissi, Italy
[3] Univ Naples Federico II, Dipartimento Sci Fis, Naples, Italy
[4] Univ Naples Federico II, Dipartimento Sci Terra, Naples, Italy
[5] CNR, I-80125 Naples, Italy