--- /dev/null
+## Copyright (C) 2009, 2010, 2012 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 SS models.
+
+## Author: Lukas Reichlin <lukas.reichlin@gmail.com>
+## Created: October 2009
+## Version: 0.3
+
+function sys = __sys_inverse__ (sys)
+
+ a = sys.a;
+ b = sys.b;
+ c = sys.c;
+ d = sys.d;
+ e = sys.e;
+
+ if (! isempty (e) || rcond (d) < eps) # dss or strictly proper ss
+
+ n = rows (a);
+ m = columns (b); # p = m (square system)
+
+ if (isempty (e)) # avoid testing twice?
+ e = eye (n);
+ endif
+
+ sys.a = [a, b; c, d];
+ sys.b = [zeros(n, m); -eye(m)];
+ sys.c = [zeros(m, n), eye(m)];
+ sys.d = zeros (m);
+ sys.e = [e, zeros(n, m); zeros(m, n+m)];
+
+ sys.stname = repmat ({""}, n+m, 1);
+
+ else # proper ss
+
+ bid = b / d;
+
+ sys.a = a - bid * c;
+ sys.b = -bid;
+ sys.c = d \ c;
+ sys.d = inv (d);
+
+ endif
+
+endfunction
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