Institution: | (1) Department of Mathematics, Massachusetts Institute of Technology, 02139 Cambridge, Massachusetts, USA;(2) U.E.R. de Mathematique et d'Informatique, Universite de Bordeaux I, France;(3) Department of Mathematics, Indiana University, 47405 Bloomington, Indiana, USA;(4) Department of Mathematics, The University of Michigan, 347 West Engineering Building, 48109 Ann Arbor, Michigan, USA |