### Copyright (C) 1996 John W. Eaton
###
### 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
### 02111-1307, USA.

function x = lyap (a, b, c)

  ## Usage: x = lyap (a, b {,c})
  ##
  ## If (a, b, c) are specified, then lyap returns the solution of the
  ## Sylvester equation
  ##
  ##   a x + x b + c = 0
  ##
  ## If only (a, b) are specified, then lyap returns the solution of the 
  ## Lyapunov equation
  ##
  ##   a' x + x a + b = 0
  ##
  ## If b is not square, then lyap returns the solution of either
  ##
  ##   a' x + x a + b' b = 0 	
  ##
  ## or
  ##
  ##   a x + x a' + b b' = 0
  ##
  ## whichever is appropriate.
  ##
  ## Solves by using the Bartels-Stewart algorithm (1972).

  ## Written by A. S. Hodel (scotte@eng.auburn.edu) August 1993.
  

  if (nargin != 3 && nargin != 2)
    usage ("lyap (a, b {,c})");
  endif

  if ((n = is_square(a)) == 0)
    error ("lyap: a is not square");
  endif

  if (nargin == 2)

    ## Transform Lyapunov equation to Sylvester equation form.

    if ((m = is_square (b)) == 0)
      if ((m = rows (b)) == n)

	## solve a x + x a' + b b' = 0

	b = b * b';
	a = a';
      else 

	## Try to solve a'x + x a + b' b = 0.

	m = columns (b);
	b = b' * b;
      endif

      if (m != n)
	error ("lyap: a, b not conformably dimensioned");
      endif
    endif

    ## Set up Sylvester equation.

    c = b;
    b = a;
    a = b';

  else 

    ## Check dimensions.

    if ((m = is_square (b)) == 0)
      error ("lyap: b must be square in a sylvester equation");
    endif

    [n1, m1] = size(c);

    if (n != n1 || m != m1)
      error("lyap: a,b,c not conformably dimensioned");
    endif
  endif

  ## Call octave built-in function.

  x = syl (a, b, c);

endfunction