1 ## Copyright (C) 1993-2012 John W. Eaton
3 ## This file is part of Octave.
5 ## Octave is free software; you can redistribute it and/or modify it
6 ## under the terms of the GNU General Public License as published by
7 ## the Free Software Foundation; either version 3 of the License, or (at
8 ## your option) any later version.
10 ## Octave is distributed in the hope that it will be useful, but
11 ## WITHOUT ANY WARRANTY; without even the implied warranty of
12 ## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 ## General Public License for more details.
15 ## You should have received a copy of the GNU General Public License
16 ## along with Octave; see the file COPYING. If not, see
17 ## <http://www.gnu.org/licenses/>.
20 ## @deftypefn {Function File} {[@var{aa}, @var{bb}, @var{q}, @var{z}] =} qzhess (@var{A}, @var{B})
21 ## Compute the Hessenberg-triangular decomposition of the matrix pencil
22 ## @code{(@var{A}, @var{B})}, returning
23 ## @code{@var{aa} = @var{q} * @var{A} * @var{z}},
24 ## @code{@var{bb} = @var{q} * @var{B} * @var{z}}, with @var{q} and @var{z}
25 ## orthogonal. For example:
29 ## [aa, bb, q, z] = qzhess ([1, 2; 3, 4], [5, 6; 7, 8])
30 ## @result{} aa = [ -3.02244, -4.41741; 0.92998, 0.69749 ]
31 ## @result{} bb = [ -8.60233, -9.99730; 0.00000, -0.23250 ]
32 ## @result{} q = [ -0.58124, -0.81373; -0.81373, 0.58124 ]
33 ## @result{} z = [ 1, 0; 0, 1 ]
37 ## The Hessenberg-triangular decomposition is the first step in
38 ## Moler and Stewart's QZ@tie{}decomposition algorithm.
40 ## Algorithm taken from Golub and Van Loan,
41 ## @cite{Matrix Computations, 2nd edition}.
44 ## Author: A. S. Hodel <scotte@eng.auburn.edu>
45 ## Created: August 1993
48 function [aa, bb, q, z] = qzhess (A, B)
56 if (na != ma || na != nb || nb != mb)
57 error ("qzhess: incompatible dimensions");
60 ## Reduce to hessenberg-triangular form.
69 ## disp (["zero out aa(", num2str(i), ",", num2str(j), ")"])
71 rot = givens (aa (i-1, j), aa (i, j));
72 aa ((i-1):i, :) = rot *aa ((i-1):i, :);
73 bb ((i-1):i, :) = rot *bb ((i-1):i, :);
74 q ((i-1):i, :) = rot *q ((i-1):i, :);
76 ## disp (["now zero out bb(", num2str(i), ",", num2str(i-1), ")"])
78 rot = givens (bb (i, i), bb (i, i-1))';
79 bb (:, (i-1):i) = bb (:, (i-1):i) * rot';
80 aa (:, (i-1):i) = aa (:, (i-1):i) * rot';
81 z (:, (i-1):i) = z (:, (i-1):i) * rot';
88 bb (i, 1:(i-1)) = zeros (1, i-1);
89 aa (i, 1:(i-2)) = zeros (1, i-2);
107 %! [aa, bb, q, z] = qzhess(a, b);
108 %! assert(inv(q) - q', zeros(4), 2e-8);
109 %! assert(inv(z) - z', zeros(4), 2e-8);
110 %! assert(q * a * z, aa, 2e-8);
111 %! assert(aa .* mask, zeros(4), 2e-8);
112 %! assert(q * b * z, bb, 2e-8);
113 %! assert(bb .* mask, zeros(4), 2e-8);
126 %! mask = [0 0 0 0 0;
131 %! [aa, bb, q, z] = qzhess(a, b);
132 %! assert(inv(q) - q', zeros(5), 2e-8);
133 %! assert(inv(z) - z', zeros(5), 2e-8);
134 %! assert(q * a * z, aa, 2e-8);
135 %! assert(aa .* mask, zeros(5), 2e-8);
136 %! assert(q * b * z, bb, 2e-8);
137 %! assert(bb .* mask, zeros(5), 2e-8);