[1] CNR, Ist Applicaz Calcolo M Picone, Via Pietro Castellino 111, I-80131 Naples, Italy
[2] Univ Firenze, Dipartimento Matemat & Informat U Dini, Viale Morgagni 67-A, I-50134 Florence, Italy
[3] Charles Univ Prague, Fac Math & Phys, Dept Math Anal, Sokolovska 83, Prague 18675 8, Czech Republic
[4] Univ Bonn, Math Inst, Endenicher Allee 60, D-53115 Bonn, Germany