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Occasionally, arrays of data to be fast Fourier transformed (FFT'ed) are too large to fit in internal computer memory, and must be kept on an external storage device. This situation is especially serious for one-dimensional arrays, since they cannot be factored along the natural cleavage planes, as multi-dimensional arrays can. Two methods for FFT of such data are presented-one efficient when data storage is only slightly larger than available internal memory, and one when data is much larger. A FORTRAN program based on these methods is available.