function y = fftw(x,sign)%FFTW  Discrete Fourier transform (DFT)%   Y = FFTW(X,SIGN) sets Y to the DFT of X, computed via a%   fast Fourier transform algorithm. SIGN is the sign of the%   exponent in the definition of the DFT, and should be +1 or -1.%%   FFTW is designed for circumstances where repeated transforms%   of the same size are required.  The first call will incur a%   substantial startup cost (several seconds), but subsequent%   calls will execute very quickly.%%   Using SIGN = -1 corresponds to MATLAB's FFTN function.%   Using SIGN = +1 corresponds to an unnormalized IFFTN:%%      FFTW(X,+1) is the same as IFFTN(X) * PROD(SIZE(X))%%   Thus, FFTW(FFTW(X,-1),+1) / PROD(SIZE(X)) equals X.%%   The input array X may have any dimensionality.  If X%   is multi-dimensional, then a true multi-dimensional DFT%   will be computed.%%   This is an interface to the FFTW C library, which is%   described at:  http://www.fftw.org%%   See also: FFT, IFFT, FFT2, IFFT2, FFTN, IFFTN, FFTSHIFT