--- /dev/null
+## Copyright (C) 2009 Lukas F. Reichlin
+##
+## This file is part of LTI Syncope.
+##
+## LTI Syncope 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.
+##
+## LTI Syncope 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 LTI Syncope. If not, see <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {@var{sys} =} lft (@var{sys1}, @var{sys2})
+## @deftypefnx {Function File} {@var{sys} =} lft (@var{sys1}, @var{sys2}, @var{nu}, @var{ny})
+## Linear fractional tranformation, also known as Redheffer star product.
+##
+## @strong{Inputs}
+## @table @var
+## @item sys1
+## Upper LTI model.
+## @item sys2
+## Lower LTI model.
+## @item nu
+## The last nu inputs of @var{sys1} are connected with the first nu outputs of @var{sys2}.
+## If not specified, @code{min (m1, p2)} is taken.
+## @item ny
+## The last ny outputs of @var{sys1} are connected with the first ny inputs of @var{sys2}.
+## If not specified, @code{min (p1, m2)} is taken.
+## @end table
+##
+## @strong{Outputs}
+## @table @var
+## @item sys
+## Resulting LTI model.
+## @end table
+##
+## @strong{Block Diagram}
+## @example
+## @group
+## .............sys..............
+## : +--------+ :
+## w1 ------------>| |------------> z1
+## : | sys1 | :
+## : u +---->| |-----+ y :
+## : | +--------+ | : Lower LFT
+## : | | :
+## : | +--------+ | : lft (sys1, sys2)
+## : +-----| sys2 |<----+ :
+## : +--------+ :
+## :............................:
+## @end group
+## @end example
+## @example
+## @group
+## .............sys..............
+## : +--------+ :
+## : u +---->| sys1 |-----+ y :
+## : | +--------+ | : Upper LFT
+## : | | :
+## : | +--------+ | : lft (sys1, sys2)
+## : +-----| |<----+ :
+## : | sys2 | :
+## z2 <------------| |<------------ w2
+## : +--------+ :
+## :............................:
+## @end group
+## @end example
+## @example
+## @group
+## .............sys..............
+## : +--------+ :
+## w1 ------------>| |------------> z1
+## : | sys1 | :
+## : u +---->| |-----+ y :
+## : | +--------+ | :
+## : | | : lft (sys1, sys2, nu, ny)
+## : | +--------+ | :
+## : +-----| |<----+ :
+## : | sys2 | :
+## z2 <------------| |<------------ w2
+## : +--------+ :
+## :............................:
+## @end group
+## @end example
+## @end deftypefn
+
+## Author: Lukas Reichlin <lukas.reichlin@gmail.com>
+## Created: October 2009
+## Version: 0.2
+
+function sys = lft (sys1, sys2, nu, ny)
+
+ if (nargin != 2 && nargin != 4)
+ print_usage ();
+ endif
+
+ ## object conversion done by sys_group if necessary
+
+ [p1, m1] = size (sys1);
+ [p2, m2] = size (sys2);
+
+ nu_max = min (m1, p2);
+ ny_max = min (m2, p1);
+
+ if (nargin == 2) # sys = lft (sys1, sys2)
+ nu = nu_max;
+ ny = ny_max;
+ else # sys = lft (sys1, sys2, nu, ny)
+ if (! is_real_scalar (nu) || nu < 0)
+ error ("lft: argument nu must be a positive integer");
+ endif
+
+ if (! is_real_scalar (ny) || ny < 0)
+ error ("lft: argument ny must be a positive integer");
+ endif
+
+ if (nu > nu_max)
+ error ("lft: argument nu (%d) must be at most %d", nu, nu_max);
+ endif
+
+ if (ny > ny_max)
+ error ("lft: argument ny (%d) must be at most %d", ny, ny_max);
+ endif
+ endif
+
+ M11 = zeros (m1, p1);
+ M12 = [zeros(m1-nu, p2); eye(nu), zeros(nu, p2-nu)];
+ M21 = [zeros(ny, p1-ny), eye(ny); zeros(m2-ny, p1)];
+ M22 = zeros (m2, p2);
+
+ M = [M11, M12; M21, M22];
+
+ in_idx = [1 : (m1-nu), m1 + (ny+1 : m2)];
+ out_idx = [1 : (p1-ny), p1 + (nu+1 : p2)];
+
+ sys = __sys_group__ (sys1, sys2);
+ sys = __sys_connect__ (sys, M);
+ sys = __sys_prune__ (sys, out_idx, in_idx);
+
+ [p, m] = size (sys);
+
+ if (m == 0)
+ warning ("lft: resulting system has no inputs");
+ endif
+
+ if (p == 0)
+ warning ("lft: resulting system has no outputs");
+ endif
+
+endfunction