CHINESE PHYSICS LETTERS, cilt.25, sa.1, ss.352, 2008 (SCI İndekslerine Giren Dergi)
The results of ours are correct. There are misprints in the sign of Eq. (11) and the factor (n - 1) in Eq. (12) in our paper [Chin. Phys. Lett. 24 (2007) 2185]. We did not study special values of n = 1 and n = -3.