1. V.I.Romanovskiy Institute of Mathematics, Uzbekistan Academy of Sciences, Tashkent, Uzbekistan
2. Department of Mathematics, Karakalpak State University, Nukus, Uzbekistan
3. Department of Mathematics, National University of Uzbekistan, Tashkent, Uzbekistan
4. Department of Mathematics, Wilfrid Laurier University, Waterloo, Canada
5. College of Mathematics and Information Science, Hebei Normal (Teachers) University, Shijiazhuang People's Republic of China