1. a Faculty of Mathematics and Computer Science , Babeş-Bolyai University , 1 M. Kogălniceanu Str., 400084 Cluj-Napoca , Romania
2. b Department of Mathematics , Indian Institute of Technology Kharagpur , Kharagpur 721 302 , India
3. c Institut für Angewandte Analysis und Numerische Simulation, Universität Stuttgart , Pfaffenwaldring 57, 70569 Stuttgart , Germany