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+## Copyright (C) 1993, 1994, 1995 Auburn University.  All rights reserved.
+##
+## This file is part of Octave.
+##
+## Octave 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, or (at your option) any
+## later version.
+##
+## Octave 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 Octave; see the file COPYING.  If not, write to the Free
+## Software Foundation, 59 Temple Place, Suite 330, Boston, MA 02111 USA.
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {@var{retval} =} is_stable (@var{a}@{,@var{tol},@var{dflg}@})
+## @deftypefnx {Function File} {@var{retval} =} is_stable (@var{sys}@{,@var{tol}@})
+## Returns retval = 1 if the matrix @var{a} or the system @var{sys}
+## is stable, or 0 if not.
+##
+## @strong{Inputs}
+## @table @var
+## @item  tol
+## is a roundoff paramter, set to 200*@var{eps} if omitted.
+## @item dflg
+## Digital system flag (not required for system data structure):
+## @table @code
+## @item @var{dflg} != 0
+## stable if eig(a) in unit circle
+##
+## @item @var{dflg} == 0
+## stable if eig(a) in open LHP (default)
+## @end table
+## @end table
+## @end deftypefn
+## @seealso{size, rows, columns, length, is_matrix, is_scalar, is_vector
+## is_observable, is_stabilizable, is_detectable, krylov, and krylovb}
+
+## Author: A. S. Hodel <a.s.hodel@eng.auburn.edu>
+## Created: August 1993
+## Updated by John Ingram (ingraje@eng.auburn.edu) July, 1996 for systems
+## Updated to simpler form by a.s.hodel 1998
+
+function retval = is_stable (a, tol, disc)
+
+  if( (nargin < 1) | (nargin > 3) )   usage("is_stable(a {,tol,disc})");
+  elseif(is_struct(a))
+    ## system was passed
+    if(nargin < 3)                      disc = is_digital(a);
+    elseif(disc != is_digital(a))
+      warning("is_stable: disc =%d does not match system",disc)
+    endif
+    sys = sysupdate(a,"ss");
+    a = sys2ss(sys);
+  else
+    if(nargin < 3)              disc = 0;               endif
+    if(is_square(a) == 0)
+      error("A(%dx%d) must be square",rows(A), columns(A));
+    endif
+  endif
+
+  if(nargin < 2)                tol = 200*eps;
+  elseif( !is_scalar(tol) )
+    error("is_stable: tol(%dx%d) must be a scalar",rows(tol),columns(tol));
+  endif
+
+  l = eig(a);
+  if(disc)      nbad = sum(abs(l)*(1+tol) > 1);
+  else          nbad = sum(real(l)+tol > 0);            endif
+  retval = (nbad == 0);
+
+endfunction