1. Department of Mathematics & Computer Science, Çankaya University, Ogretmenler Cad. 14 06530, Balgat, Ankara, Turkey
2. Department of Mathematics & Computer Science, University of Craiova, DAL, Tudor Vladimirescu 26, 200534 Craiova, Romania
3. Department of Mathematical Sciences, Florida Institute of Technology, Melbourne, FL 32901, USA