## 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, 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