Institution: | (1) EPFL Mathematics Institute, CH-1015 Lausanne, Switzerland;(2) Supported by the European DONET program, TMR ERB, FMRX-CT98-0202;(3) Dipartimento di Informatica Sistemi e Produzione, University Tor Vergata, Rome, Italy;(4) Department of Mathematics/IMO, University of Magdeburg, Universitätsplatz, 2 39106 Magdeburg, Germany |