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The discrete Hartley transform (DHT) of a 1d real array *X* of size
*n* computes a real array *Y* of the same size, where:

FFTW computes an unnormalized transform, in that there is no coefficient
in front of the summation in the DHT. In other words, applying the
transform twice (the DHT is its own inverse) will multiply the input by
*n*.