1 ## Copyright (C) 2000-2012 Paul Kienzle
3 ## This file is part of Octave.
5 ## Octave is free software; you can redistribute it and/or modify it
6 ## under the terms of the GNU General Public License as published by
7 ## the Free Software Foundation; either version 3 of the License, or (at
8 ## your option) any later version.
10 ## Octave is distributed in the hope that it will be useful, but
11 ## WITHOUT ANY WARRANTY; without even the implied warranty of
12 ## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 ## General Public License for more details.
15 ## You should have received a copy of the GNU General Public License
16 ## along with Octave; see the file COPYING. If not, see
17 ## <http://www.gnu.org/licenses/>.
20 ## @deftypefn {Function File} {@var{p} =} factor (@var{q})
21 ## @deftypefnx {Function File} {[@var{p}, @var{n}] =} factor (@var{q})
23 ## Return prime factorization of @var{q}. That is,
24 ## @code{prod (@var{p}) == @var{q}} and every element of @var{p} is a prime
25 ## number. If @code{@var{q} == 1}, return 1.
27 ## With two output arguments, return the unique primes @var{p} and
28 ## their multiplicities. That is, @code{prod (@var{p} .^ @var{n}) ==
33 ## Author: Paul Kienzle
35 ## 2002-01-28 Paul Kienzle
36 ## * remove recursion; only check existing primes for multiplicity > 1
37 ## * return multiplicity as suggested by Dirk Laurie
38 ## * add error handling
40 function [x, n] = factor (q)
46 if (! isscalar (q) || q != fix (q))
47 error ("factor: Q must be a scalar integer");
50 ## Special case of no primes less than sqrt(q).
58 ## There is at most one prime greater than sqrt(q), and if it exists,
59 ## it has multiplicity 1, so no need to consider any factors greater
60 ## than sqrt(q) directly. [If there were two factors p1, p2 > sqrt(q),
61 ## then q >= p1*p2 > sqrt(q)*sqrt(q) == q. Contradiction.]
62 p = primes (sqrt (q));
64 ## Find prime factors in remaining q.
65 p = p (rem (q, p) == 0);
67 ## Can't be reduced further, so q must itself be a prime.
76 ## Determine muliplicity.
78 idx = find ([0, x] != [x, 0]);
79 x = x(idx(1:length(idx)-1));
86 %! assert(factor(1),1);
90 %! assert(all(isprime(p)));
92 %! assert(prod(p.^n),i);
93 %! assert(all([0,p]!=[p,0]));