1. a Department of Physics and Mathematics , University of Joensuu , P. O. Box 111, Joensuu 80101 , Finland
2. b Department of Mathematics and Statistics , College of Science, Sultan Qaboos University , P. O. Box 36, PC 123, Al Khodh, Muscat , Sultanate of Oman
3. c Ufa State Aviation Technical University , Ufa 450025 , Russia