1. Czech Technical University in Prague, Faculty of Electrical Engineering, Technicka 2, 166 27, Prague 6, Czech Republic
2. Ch. Abdirov 1, Department of Mathematics, Karakalpak State University, Nukus 230113, Uzbekistan
3. Departamento de Análisis Matemático, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain
4. Department of Mathematics, University of California, Irvine, California 92697-3875, USA