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+## Copyright (C) 2000 Paul Kienzle
+##
+## This program is free software; you can redistribute it and/or modify
+## it under the terms of the GNU General Public License as published by
+## the Free Software Foundation; either version 2 of the License, or
+## (at your option) any later version.
+##
+## This program is distributed in the hope that it will be useful,
+## but WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+## GNU General Public License for more details.
+##
+## You should have received a copy of the GNU General Public License
+## along with this program; if not, write to the Free Software
+## Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {[@var{q}]} polygcd (@var{b}, @var{a}, @var{tol})
+##
+## Find greatest common divisor of two polynomials.  This is equivalent
+## to the polynomial found by multiplying together all the common roots.
+## Together with deconv, you can reduce a ratio of two polynomials.
+## Tolerance defaults to 
+## @example 
+## sqrt(eps).
+## @end example
+##  Note that this is an unstable
+## algorithm, so don't try it on large polynomials.
+##
+## Example
+## @example
+##    polygcd(poly(1:8),poly(3:12)) - poly(3:8)
+##    deconv(poly(1:8),polygcd(poly(1:8),poly(3:12))) - poly(1:2)
+## @end example
+## @end deftypefn
+##
+## @seealso{poly, polyinteg, polyderiv, polyreduce, roots, conv, deconv,
+## residue, filter, polyval, and polyvalm}
+
+function x = polygcd(b,a,tol)
+  if (nargin<2 || nargin>3)
+    usage("x=polygcd(b,a [,tol])");
+  endif
+  if (nargin<3), tol=sqrt(eps); endif
+  if (length(a)==1 || length(b)==1)
+    if a==0, x=b;
+    elseif b==0, x=a;
+    else x=1;
+    endif
+    return;
+  endif
+  a = a./a(1);
+  while (1)
+    [d, r] = deconv(b, a);
+    nz = find(abs(r)>tol);
+    if isempty(nz)
+      x = a; 
+      return;
+    else
+      r = r(nz(1):length(r));
+    endif
+    b = a;
+    a = r./r(1);
+  endwhile
+endfunction
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