1. a Department of Mathematics and Statistics , King Fahd University of Petroleum and Minerals , Dhahran , Saudi Arabia
2. b Lobachevsky Institute of Mathematics and Mechanics , Kazan (Volga Region) Federal University , Kazan , Tatarstan , Russia
3. c Department of Mathematics , Hanover College , Hanover , Indiana , USA
4. d Institute of Mathematics , Hanoi , Vietnam