[1] Imperial Coll London, Dept Math, London SW7 2AZ, England
[2] Univ Paris 09, PSL Res Univ, Ctr Rech Math Decis, CNRS,UMR 7534, Pl Lattre de Tassigny, F-75775 Paris 16, France
[3] Ludwig Maximilians Univ Munchen, Math Inst, Theresienstr 39, D-80333 Munich, Germany
[4] CALTECH, Dept Math, Pasadena, CA 91125 USA
[5] CALTECH, Dept Comp & Math Sci, 1200 E Calif Blvd MC 305-16, Pasadena, CA 91125 USA