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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},@var{U}] =} is_observable (@var{a}, @var{c}@{,@var{tol}@})
+## @deftypefnx {Function File} {[@var{retval},@var{U}] =} is_observable (@var{sys}@{, @var{tol}@})
+## Logical check for system observability.
+##
+## Default: tol = 10*norm(a,'fro')*eps
+##
+## Returns 1 if the system @var{sys} or the pair (@var{a},@var{c}) is
+## observable, 0 if not.
+##
+## @strong{See} @code{is_controllable} for detailed description of arguments
+## and default values.
+## @end deftypefn
+## @seealso{size, rows, columns, length, is_matrix, is_scalar, and is_vector}
+
+## Author: A. S. Hodel <a.s.hodel@eng.auburn.edu>
+## Created: August 1993
+## Updated by John Ingram (ingraje@eng.auburn.edu) July 1996.
+
+function [retval, U] = is_observable (a, c, tol)
+
+  if( nargin < 1)
+    usage("[retval,U] = is_observable(a , c {, tol})");
+  elseif(is_struct(a))
+    ## system form
+    if(nargin == 2)
+      tol = c;
+    elseif(nargin > 2)
+      usage("[retval,U] = is_observable(sys {, tol})");
+    endif
+    [a,b,c] = sys2ss(a);
+  elseif(nargin > 3)
+    usage("[retval,U] = is_observable(a , c {, tol})");
+  endif
+  if(exist("tol"))
+    [retval,U] = is_controllable (a', c', tol);
+  else
+    [retval,U] = is_controllable (a', c');
+  endif
+
+endfunction
+