1. Department of Mathematics, TOBB University of Economics and Technology, Ankara 06560, Turkey
2. Universidad Politécnica de Uruapan, Carretera Carapan, 60120 Uruapan Michoacán, México
3. Theoretical Astrophysics, IAAT, Eberhard Karls University of Tübingen, Tübingen 72076, Germany
4. Department of Mathematics, Wayne State University, 656 W. Kirby, Detroit, Michigan 48202, USA