1. Departamento de Física, Universidade Federal de Pernambuco, Recife, PE 50670-901, Brazil
2. Department of Physics, University of Zanjan, P.O. Box 45371-38791, Zanjan, Iran
3. Research Institute for Astronomy and Astrophysics of Maragha (RIAAM), University of Maragheh, P.O. Box 55136-553, Maragheh, Iran
4. Department of Applied Mathematics, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada