1. Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Am Mühlenberg 1, D-14476 Potsdam, Germany
2. Institute of Radiation Problems ANAS, B. Vahabzade 9, AZ1143 Baku, Azerbaijan
3. Department of Mathematics, Khazar University, Mehseti St. 41, AZ1096, Baku, Azerbaijan
4. Department of Physics, University of Connecticut, Storrs, CT 06269, USA