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+## Copyright (C) 1996, 1998 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{outputs} =} tf2ss (@var{inputs})
+## @format
+## Conversion from tranfer function to state-space.
+## The state space system
+##       .
+##       x = Ax + Bu
+##       y = Cx + Du
+##
+## is obtained from a transfer function
+##
+##                 num(s)
+##           G(s)=-------
+##                 den(s)
+##
+## via the function call [a,b,c,d] = tf2ss(num,den).
+## The vector 'den' must contain only one row, whereas the vector 'num'
+## may contain as many rows as there are outputs of the system 'y'.
+## The state space system matrices obtained from this function will be
+## in controllable canonical form as described in "Modern Control Theory",
+## [Brogan, 1991].
+##
+##
+## @end format
+## @end deftypefn
+
+## Author: R. Bruce Tenison <btenison@eng.auburn.edu>
+## Created: June 22, 1994
+## mod A S Hodel July, Aug  1995
+
+function [a, b, c, d] = tf2ss (num, den)
+
+  if(nargin != 2)        error("tf2ss: wrong number of input arguments")
+  elseif(isempty(num))   error("tf2ss: empty numerator");
+  elseif(isempty(den))   error("tf2ss: empy denominator");
+  elseif(!is_vector(num))
+    error(sprintf("num(%dx%d) must be a vector",rows(num),columns(num)));
+  elseif(!is_vector(den))
+    error(sprintf("den(%dx%d) must be a vector",rows(den),columns(den)));
+  endif
+
+  ## strip leading zeros from num, den
+  nz = find(num != 0);
+  if(isempty(nz)) num = 0;
+  else num = num(nz(1):length(num));         endif
+  nz = find(den != 0);
+  if(isempty(nz)) error("denominator is 0.");
+  else den = den(nz(1):length(den));         endif
+
+  ## force num, den to be row vectors
+  num = vec(num)';        den = vec(den)';
+  nn = length(num);       nd = length(den);
+  if(nn > nd) error(sprintf("deg(num)=%d > deg(den)= %d",nn,nd)); endif
+
+   ## Check sizes
+   if (nd == 1)      a = []; b = []; c = []; d = num(:,1) / den(1);
+   else
+    ## Pad num so that length(num) = length(den)
+    if (nd-nn > 0) num = [zeros(1,nd-nn), num]; endif
+
+    ## Normalize the numerator and denominator vector w.r.t. the leading
+    ## coefficient
+    d1 = den(1);    num = num / d1;    den = den(2:nd)/d1;
+    sw = nd-1:-1:1;
+
+    ## Form the A matrix
+    if(nd > 2)      a = [zeros(nd-2,1),eye(nd-2,nd-2);-den(sw)];
+    else            a = -den(sw);                                endif
+
+    ## Form the B matrix
+    b = zeros(nd-1,1);           b(nd-1,1) = 1;
+
+    ## Form the C matrix
+    c = num(:,2:nd)-num(:,1)*den;        c = c(:,sw);
+
+    ## Form the D matrix
+    d = num(:,1);
+  endif
+
+endfunction