Affiliation: | (1) Department of Mathematics, Carnegie-Mellon University, 15213 Pittsburgh, PA, USA;(2) Present address: Department of Mathematics, University of Minnesota, 55455 Minneapolis, MN, USA;(3) Department of Mathematics, Rutgers University, 08903 New Brunswick, NJ, USA;(4) Bell Communications Research, 07960 Morristown, NJ, USA |