Affiliation: | a Mathematical Centre, P.O. Box 4079, 1009 AB, Amsterdam, The Netherlands b Dept. Math., Erasmus Univ. Rotterdam, P.O. Box 1738, 3000 DR, Rotterdam, The Netherlands Dept. Electrical Engineering, University of Texas, Austin, TX 78712, USA Dept. Mathematics, Rutgers University, New Brunswick, NJ 08903, USA |