## functions in convol.i - f

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