y Format</a>.
</li><li>- FFTW is generally best at handling sizes of the form
2<sup>a</sup> 3<sup>b</sup> 5<sup>c</sup> 7<sup>d</sup>
        11<sup>e</sup> 13<sup>f</sup>,
where <em>e+f</em> is either <em>0</em> or <em>1</em>, and the other exponents
are arbitrary.  Other sizes are computed by means of a slow,
general-purpose algorithm (which nevertheless retains <i>O</i>(<i>n</i>&nbsp;log&nbsp;<i>n</i>)
 performance even for prime sizes).  (It is possible to customize FFTW
for different array sizes; see <