1. Institute of Mathematics, Uzbekistan Academy of Science, University Str., 9, Olmazor District, Tashkent 100174, Uzbekistan
2. AU Engineering School, Akfa University, 264, Milliy Bog Str., Tashkent 111221, Uzbekistan
3. Department of Higher Mathematics, University of Information Technologies, 108 Amir Temur Avenue, Tashkent 100200, Uzbekistan
4. Department of Mathematics, University of New Haven, 300 Boston Post Road, West Haven, CT 06516, USA