1. Department of Mathematics, Cankaya University, Öčretmenler Cad., 1406530, Ankara 6400, Turkey; Institute of Space Sciences, Magurele, Bucharest 77125, Romania e-mail:
2. Department of Mathematics, Firat University, Elazič 23119, Turkey e-mail:
3. Department of Mathematics, Firat University, Elazič 23119, Turkey; Department of Mathematics, Federal University Dutse, Jigawa 7156, Nigeria e-mail: