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Version:
4.4.5.dfsg-3ubuntu2 ▾
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<HTML>
<HEAD>
<TITLE>Prime factors</TITLE>
</HEAD>
<BODY bgcolor="#FFFFFF" fgcolor="#000000">
<p>
<font size="+1" color="green"><B>Prime factors</B></font></P>
<p>
Suppose that <code>LEN(x) = 2N</code>. The largest prime factor of <code>2N</code> must be
<code>≤23</code>, and there must be no more than <code>11</code> distinct prime factors.
The product of the square-free prime factors of <code>2N</code> must be <code>≤210</code>.
The calculation speed is enhanced by using a value of <code>2N</code> with small prime
factors, particularly powers of <code>2</code>. For reference, the prime numbers less than
<code>1000</code> are listed below.</p>
<p>
<pre>
2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59
61 67 71 73 79 83 89 97 101 103 107 109 113 127 131 137 139
149 151 157 163 167 173 179 181 191 193 197 199 211 223 227 229 233
239 241 251 257 263 269 271 277 281 283 293 307 311 313 317 331 337
347 349 353 359 367 373 379 383 389 397 401 409 419 421 431 433 439
443 449 457 461 463 467 479 487 491 499 503 509 521 523 541 547 557
563 569 571 577 587 593 599 601 607 613 617 619 631 641 643 647 653
659 661 673 677 683 691 701 709 719 727 733 739 743 751 757 761 769
773 787 797 809 811 821 823 827 829 839 853 857 859 863 877 881 883
887 907 911 919 929 937 941 947 953 967 971 977 983 991 997
</pre></p>
<p>
For example, <code>2N = 202 = 2*101</code> is not allowed, and <code>2N = 402 = 2*3*67</code>
is also not allowed. If <code>2N</code> does not satisfy the above restrictions, the input
vector can be padded out, usually with zeroes, to an even length whose prime factors do not
exceed <code>23</code>. The <code><a href="pfactors.htm">PFACTORS</a>
</code> function returns the prime factors of a constant or scalar.</p>
<P>
<a href="fftS03.htm"><img align="top" border="0" src="../shadow_left.gif">
<font size="+1" color="olive">Restrictions</font></a><br />
<a href="fftS05.htm"><img align="top" border="0" src="../shadow_right.gif">
<font size="+1" color="olive">Example</font></a></P>
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