CHINESE PHYSICS LETTERS, vol.25, no.1, pp.352, 2008 (SCI-Expanded)
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.