1. Departamento de Matemáticas; Instituto Tecnológico Autónomo de México; Río Hondo #1, Col. Progreso Tizapán México, DF 01080 México
2. Bauhaus-Universität Weimar; Institut für Mathematik/Physik; Coudraystr. 13B Weimar D-99421 Germany
3. Department of Mathematics, Faculty of Science and Technology; University of Macau; Macau