Affiliation: | (1) Institute of Mathematics, Wrocław University of Technology, Wyspiańskiego 27, 50-370 Wrocław, Poland (e-mail: nowak@im.pwr.wroc.pl), PL;(2) Departamento de Matematicas, Universidad de Sonora, Blvd. Transversal y Rosales s/n Hermosillo, Sonora, Mexico (e-mail: ovega@fisica.uson.mx), MX |