1. Department of Mathematics University of Brasília Brasília–DF 70910‐900 Brazil
2. Institute of Cybernetics of the Georgian Technical University Sandro Euli Str. 5, 0186 Tbilisi Georgia
3. Departamento de Matemáticas, Esc. Sup. de Enx. Informática Campus de Ourense, Universidade de Vigo Ourense E–32004 Spain
4. Faculty of Engineering Vrije Universiteit Brussel Pleinlaan 2 Brussel B–1050 Belgium