1. Departamento de Física, Universidade Estadual de Londrina, Caixa Postal 6001, Londrina-Paraná, Brazil
2. Department of Physics, University of Helsinki, P. O. Box 64, FI-00014 Helsinki, Finland
3. Department of Mathematics, Heriot-Watt University, Colin Maclaurin Building, Riccarton, Edinburgh EH14 4AS, UK
4. Maxwell Institute for Mathematical Sciences, Edinburgh, UK
5. The Tait Institute, Edinburgh, UK