--- /dev/null
+## Copyright (C) 2009, 2011 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 -*-
+## Inversion of TF models.
+
+## Author: Lukas Reichlin <lukas.reichlin@gmail.com>
+## Created: October 2009
+## Version: 0.2
+
+function sys = __sys_inverse__ (sys)
+
+ nvec = size (sys);
+ num = sys.num;
+ den = sys.den;
+
+ if (all (nvec == 1)) # SISO
+ if (num{1,1} == 0) # catch case num = 0
+ sys.num(1,1) = tfpoly (0);
+ sys.den(1,1) = tfpoly (1);
+ else
+ sys.num = den;
+ sys.den = num;
+ endif
+ elseif (all (nvec == 2)) # 2x2 MIMO
+ sys.num(1,1) = -den{1,1}*den{1,2}*den{2,1}*num{2,2};
+ sys.num(1,2) = den{1,1}*den{2,1}*den{2,2}*num{1,2};
+ sys.num(2,1) = den{1,1}*den{1,2}*den{2,2}*num{2,1};
+ sys.num(2,2) = -den{1,2}*den{2,1}*den{2,2}*num{1,1};
+ sys.den(:) = den{1,1}*den{2,2}*num{1,2}*num{2,1} - den{1,2}*den{2,1}*num{1,1}*num{2,2};
+ else
+ ## I've calculated 3x3 systems with sage but the formula is quite long
+ [num, den] = tfdata (inv (ss (sys)), "tfpoly");
+ sys.num = num;
+ sys.den = den;
+ endif
+
+endfunction