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.
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12 ## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 ## General Public License for more details.
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20 ## @deftypefn {Function File} {@var{q} =} polygcd (@var{b}, @var{a})
21 ## @deftypefnx {Function File} {@var{q} =} polygcd (@var{b}, @var{a}, @var{tol})
23 ## Find the greatest common divisor of two polynomials. This is equivalent
24 ## to the polynomial found by multiplying together all the common roots.
25 ## Together with deconv, you can reduce a ratio of two polynomials.
26 ## The tolerance @var{tol} defaults to @code{sqrt(eps)}.
28 ## @strong{Caution:} This is a numerically unstable algorithm and should not
29 ## be used on large polynomials.
35 ## polygcd (poly (1:8), poly (3:12)) - poly (3:8)
36 ## @result{} [ 0, 0, 0, 0, 0, 0, 0 ]
37 ## deconv (poly (1:8), polygcd (poly (1:8), poly (3:12))) - poly(1:2)
38 ## @result{} [ 0, 0, 0 ]
41 ## @seealso{poly, roots, conv, deconv, residue}
44 function x = polygcd (b, a, tol)
46 if (nargin == 2 || nargin == 3)
48 if (isa (a, "single") || isa (b, "single"))
49 tol = sqrt (eps ("single"));
54 if (length (a) == 1 || length (b) == 1)
65 [d, r] = deconv (b, a);
66 nz = find (abs (r) > tol);
71 r = r(nz(1):length(r));
85 %! poly1 = [1 6 11 6]; % (x+1)(x+2)(x+3)
86 %! poly2 = [1 3 2]; % (x+1)(x+2)
87 %! poly3 = polygcd (poly1, poly2);
88 %! assert (poly3, poly2, sqrt (eps))
91 %! assert (polygcd (poly(1:8), poly(3:12)), poly(3:8), sqrt (eps))
94 %! assert (deconv (poly(1:8), polygcd (poly(1:8), poly(3:12))), poly(1:2), sqrt (eps))
98 %! p = (unique (randn (10, 1)) * 10).';
101 %! assert (polygcd (poly (-p1), poly (-p2)), poly (- intersect (p1, p2)), sqrt (eps))