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## Copyright (C) 1993, 1994, 1995, 2000, 2002, 2004, 2005, 2006, 2007
## Auburn University. All rights reserved.
##
##
## 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 3 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; see the file COPYING. If not, see
## <http://www.gnu.org/licenses/>.
## -*- texinfo -*-
## @deftypefn {Function File} {} is_stable (@var{a}, @var{tol}, @var{dflg})
## @deftypefnx {Function File} {} is_stable (@var{sys}, @var{tol})
## Returns 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 parameter, set to 200*@code{eps} if omitted.
## @item dflg
## Digital system flag (not required for system data structure):
## @table @code
## @item @var{dflg} != 0
## stable if eig(a) is in the unit circle
##
## @item @var{dflg} == 0
## stable if eig(a) is in the open LHP (default)
## @end table
## @end table
## @seealso{size, rows, columns, length, ismatrix, isscalar, isvector, is_observable, is_stabilizable, is_detectable, krylov, krylovb}
## @end deftypefn
## 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)
print_usage ();
elseif (isstruct (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 (issquare (a) == 0)
error ("A(%dx%d) must be square", rows (A), columns (A));
endif
endif
if (nargin < 2)
if (isa (a, "single"))
tol = 200 * eps("single");
else
tol = 200 * eps;
endif
elseif (! isscalar (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
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