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fft_good |
                 fft_good(n)

         returns the smallest number of the form 2^x*3^y*5^z greater
         than or equal to n.  An fft of this length will be much faster
         than a number with larger prime factors; the speed difference
         can be an order of magnitude or more.
         For n>100, the worst cases result in a little over a 11% increase
         in n; for n>1000, the worst are a bit over 6%; still larger n are
         better yet.  The median increase for n<=10000 is about 1.5%.
SEE ALSO: | fft, fft_setup, convol