Affiliation: | a Centro de Investigación Operativa, Universidad M. Hernández, E-03202 Elche (Alicante), Spain b Faculty of Mathematics and Informatics, A. Mickiewicz University, 61-614 Poznań, Poland c Departamento de Matemática Aplicada y IMPA, Universidad Politécnica, E-46022 Valencia, Spain d Department of Mathematics, University of Florida, PO Box 11805, Gainesville, FL 32611-8105, USA |