\[S(\alpha,\beta,n)=\frac{e^{i\beta}}{e^{i\beta}-1}\left(e^{i(n-1)\beta}n^{%
\alpha}-1+\sum_{j=1}^{n-1}e^{ij\beta}\left(j^{\alpha}-(j+1)^{\alpha}\right)%
\right).\]