[1] Swiss Fed Inst Technol, ETH, Zurich, Switzerland
[2] Univ Neuchatel, CH-2000 Neuchatel, Switzerland
[3] Univ London Birkbeck Coll, London WC1E 7HX, England
[4] Univ Bologna, Dipartimento Matemat, I-40126 Bologna, Italy
[5] CNR, IMATI, I-27100 Pavia, Italy