Affiliation: | (1) Department of Mathematics, Virginia Polytechic Institute and State University, 24061-0123 Blacksburg, VA;(2) Departamento de Matemática, Instituto Superior Técnico, Av. Rovisco Pais, 1096 Lisboa, Portugal;(3) Department of Mathematics, The College of William & Mary, P.O. Box 8795, 23187-8795 Williamsburg, VA |