1. Department of Mathematics, Chelyabinsk State University, Chelyabinsk, Russia
2. Department of Computer Science, CICESE, Carretera Ensenada-Tijuana 3918, 22860 Ensenada, BC, Mexico
3. Facultad de Ciencias, Universidad Autonoma de Baja California, Carretera Tijuana-Ensenada, No. 3917, 22860 Ensenada, BC, Mexico