[1] Univ Genoa, Dipartimento Ingn Meccan Energet Gest & Trasporti, Via Opera Pia 15, I-16145 Genoa, Italy
[2] Kyushu Univ, Fac Math, Nishi Ku, 744 Motooka, Fukuoka 8190395, Japan
[3] Univ Vienna, Fac Math, Oskar Morgenstern Pl 1, A-1090 Vienna, Austria
[4] CNR, Ist Matemat Appl & Tecnol Informat E Magenes, V Ferrata 1, I-27100 Pavia, Italy