functions in convol.i - f

    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%.