Rader's algorithm, exploiting the existence of a generator for the multiplicative group modulo prime "N", expresses a DFT of prime size "N" as a cyclic convolution of (composite) size "N" − 1, which can then be computed by a pair of ordinary FFTs via the convolution theorem (although Winograd uses other convolution methods). Another prime-size FFT is due to L. I. Bluestein, and is sometimes called the chirp-z algorithm; it also re-expresses a DFT as a convolution, but this time of the "same" size (which can be zero-padded to a power of two and evaluated by radix-2 Cooley–Tukey FFTs, for example), via the identity