Let X[n] be a DT signal with period N
DFT $ X [k] = \sum_{k=0}^{N-1} x[n].e^{-J.2pi.kn/N} $
IDFT $ x [n] = (1/N) \sum_{k=0}^{N-1} X[k].e^{J.2pi.kn/N} $
