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%.