discrete sine transform using FFT i have made a subroutine to calculate 1d fft of a series of size 2^n. I need to calculate discrete sine transform of a series using the fft subroutine. I am not sure what to do.
well, you can do a little Googling and Wikipeding and find the definitions of the DST and pick one of them.  The get the definition of the DFT (the FFT is an efficient method to compute the DFT), use Euler's formula on the DST definition and see where a DFT pops out.

i might get back to this tonight and spell it out, but if you're not sure where to go next, the above (hit the links) tells you where to approach the problem.