1. Department of Mathematics, University of Oviedo, 33007 Oviedo, Spain
2. Instituto Universitario de Ciencias y Tecnologías Espaciales de Asturias (ICTEA), University of Oviedo, 33004 Oviedo, Spain
3. Department of Physics, University of Oviedo, 33007 Oviedo, Spain
4. Department of Prospecting and Exploitation of Mines, University of Oviedo, 33004 Oviedo, Spain