1. Baku State University, Z. Khalilov str. 23, AZ 1148, Baku, Azerbaijan
2. Institute of Mathematics and Mechanics, NAS of Azerbaijan, B. Vahabzadeh str. 9, AZ 1148, Baku, Azerbaijan
3. Sumgait State University, 43rd block, 1, AZ 5008, Sumgait, Azerbaijan