1. Department of Mathematics and Computer Sciences, Cankaya University, Ankara, Turkey
2. Department of Mathematics and Statistics, University of Victoria, Victoria, BC, Canada V8W 3R4
3. Department of Mathematics, University of Pune, Pune 411007, India
4. Department of Mathematics, Shanghai University, Shanghai 200444, China
5. Department of Electrical Engineering, Institute of Engineering, Polytechnic of Porto, Rua Dr. Antonio Bernardino de Almeida 431, 4200-072 Porto, Portugal