1. Departamento de Matemáticas, Universidad de Murcia, Campus de Espinardo, Aptdo. de Correos 4021, E–30100 Murcia, Spain
2. Institute of Mathematics and Informatics, Matej Bel University, Tajovského 40, SK–974 01 Banská Bystrica, Slovakia
3. Center for Mathematical Analysis, Geometry and Dynamical Systems, Instituto Superior Técnico, Avenida Rovisco Pais, P–1049 Lisboa, Portugal
4. Department of Mathematics, Faculty of Natural Sciences, Matej Bel University, Tajovského 40, SK–974 01 Banská Bystrica, Slovakia