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+## Copyright (C) 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 -*-
+## @deftypefn{Function File} {[@var{Gr}, @var{info}] =} spamodred (@var{G}, @dots{})
+## @deftypefnx{Function File} {[@var{Gr}, @var{info}] =} spamodred (@var{G}, @var{nr}, @dots{})
+## @deftypefnx{Function File} {[@var{Gr}, @var{info}] =} spamodred (@var{G}, @var{opt}, @dots{})
+## @deftypefnx{Function File} {[@var{Gr}, @var{info}] =} spamodred (@var{G}, @var{nr}, @var{opt}, @dots{})
+##
+## Model order reduction by frequency weighted Singular Perturbation Approximation (SPA).
+## The aim of model reduction is to find an LTI system @var{Gr} of order
+## @var{nr} (nr < n) such that the input-output behaviour of @var{Gr}
+## approximates the one from original system @var{G}.
+##
+## SPA is an absolute error method which tries to minimize
+## @iftex
+## @tex
+## $$ || G - G_r ||_{\\infty} = min $$
+## $$ || V \\ (G - G_r) \\ W ||_{\\infty} = min $$
+## @end tex
+## @end iftex
+## @ifnottex
+## @example
+## ||G-Gr||    = min
+##         inf
+##
+## ||V (G-Gr) W||    = min
+##               inf
+## @end example
+## @end ifnottex
+## where @var{V} and @var{W} denote output and input weightings.
+##
+##
+## @strong{Inputs}
+## @table @var
+## @item G
+## LTI model to be reduced.
+## @item nr
+## The desired order of the resulting reduced order system @var{Gr}.
+## If not specified, @var{nr} is chosen automatically according
+## to the description of key @var{'order'}.
+## @item @dots{}
+## Optional pairs of keys and values.  @code{"key1", value1, "key2", value2}.
+## @item opt
+## Optional struct with keys as field names.
+## Struct @var{opt} can be created directly or
+## by command @command{options}.  @code{opt.key1 = value1, opt.key2 = value2}.
+## @end table
+##
+## @strong{Outputs}
+## @table @var
+## @item Gr
+## Reduced order state-space model.
+## @item info
+## Struct containing additional information.
+## @table @var
+## @item info.n
+## The order of the original system @var{G}.
+## @item info.ns
+## The order of the @var{alpha}-stable subsystem of the original system @var{G}.
+## @item info.hsv
+## The Hankel singular values of the @var{alpha}-stable part of
+## the original system @var{G}, ordered decreasingly.
+## @item info.nu
+## The order of the @var{alpha}-unstable subsystem of both the original
+## system @var{G} and the reduced-order system @var{Gr}.
+## @item info.nr
+## The order of the obtained reduced order system @var{Gr}.
+## @end table
+## @end table
+##
+##
+## @strong{Option Keys and Values}
+## @table @var
+## @item 'order', 'nr'
+## The desired order of the resulting reduced order system @var{Gr}.
+## If not specified, @var{nr} is chosen automatically such that states with
+## Hankel singular values @var{info.hsv} > @var{tol1} are retained.
+##
+## @item 'left', 'output'
+## LTI model of the left/output frequency weighting @var{V}.
+## Default value is an identity matrix.
+##
+## @item 'right', 'input'
+## LTI model of the right/input frequency weighting @var{W}.
+## Default value is an identity matrix.
+##
+## @item 'method'
+## Approximation method for the L-infinity norm to be used as follows:
+## @table @var
+## @item 'sr', 's'
+## Use the square-root Singular Perturbation Approximation method.
+## @item 'bfsr', 'p'
+## Use the balancing-free square-root Singular Perturbation Approximation method.  Default method.
+## @end table
+##
+## @item 'alpha'
+## Specifies the ALPHA-stability boundary for the eigenvalues
+## of the state dynamics matrix @var{G.A}.  For a continuous-time
+## system, ALPHA <= 0 is the boundary value for
+## the real parts of eigenvalues, while for a discrete-time
+## system, 0 <= ALPHA <= 1 represents the
+## boundary value for the moduli of eigenvalues.
+## The ALPHA-stability domain does not include the boundary.
+## Default value is 0 for continuous-time systems and
+## 1 for discrete-time systems.
+##
+## @item 'tol1'
+## If @var{'order'} is not specified, @var{tol1} contains the tolerance for
+## determining the order of the reduced model.
+## For model reduction, the recommended value of @var{tol1} is
+## c*info.hsv(1), where c lies in the interval [0.00001, 0.001].
+## Default value is info.ns*eps*info.hsv(1).
+## If @var{'order'} is specified, the value of @var{tol1} is ignored.
+##
+## @item 'tol2'
+## The tolerance for determining the order of a minimal
+## realization of the ALPHA-stable part of the given
+## model.  TOL2 <= TOL1.
+## If not specified, ns*eps*info.hsv(1) is chosen.
+##
+## @item 'gram-ctrb'
+## Specifies the choice of frequency-weighted controllability
+## Grammian as follows:
+## @table @var
+## @item 'standard'
+## Choice corresponding to a combination method [4]
+## of the approaches of Enns [1] and Lin-Chiu [2,3].  Default method.
+## @item 'enhanced'
+## Choice corresponding to the stability enhanced
+## modified combination method of [4].
+## @end table
+##
+## @item 'gram-obsv'
+## Specifies the choice of frequency-weighted observability
+## Grammian as follows:
+## @table @var
+## @item 'standard'
+## Choice corresponding to a combination method [4]
+## of the approaches of Enns [1] and Lin-Chiu [2,3].  Default method.
+## @item 'enhanced'
+## Choice corresponding to the stability enhanced
+## modified combination method of [4].
+## @end table
+##
+## @item 'alpha-ctrb'
+## Combination method parameter for defining the
+## frequency-weighted controllability Grammian.
+## abs(alphac) <= 1.
+## If alphac = 0, the choice of
+## Grammian corresponds to the method of Enns [1], while if
+## alphac = 1, the choice of Grammian corresponds
+## to the method of Lin and Chiu [2,3].
+## Default value is 0.
+##
+## @item 'alpha-obsv'
+## Combination method parameter for defining the
+## frequency-weighted observability Grammian.
+## abs(alphao) <= 1.
+## If alphao = 0, the choice of
+## Grammian corresponds to the method of Enns [1], while if
+## alphao = 1, the choice of Grammian corresponds
+## to the method of Lin and Chiu [2,3].
+## Default value is 0.
+##
+## @item 'equil', 'scale'
+## Boolean indicating whether equilibration (scaling) should be
+## performed on system @var{G} prior to order reduction.
+## Default value is true if @code{G.scaled == false} and
+## false if @code{G.scaled == true}.
+## @end table
+##
+##
+## @strong{References}@*
+## [1] Enns, D.
+## Model reduction with balanced realizations: An error bound
+## and a frequency weighted generalization.
+## Proc. 23-th CDC, Las Vegas, pp. 127-132, 1984.
+##
+## [2] Lin, C.-A. and Chiu, T.-Y.
+## Model reduction via frequency-weighted balanced realization.
+## Control Theory and Advanced Technology, vol. 8,
+## pp. 341-351, 1992.
+##
+## [3] Sreeram, V., Anderson, B.D.O and Madievski, A.G.
+## New results on frequency weighted balanced reduction
+## technique.
+## Proc. ACC, Seattle, Washington, pp. 4004-4009, 1995.
+##
+## [4] Varga, A. and Anderson, B.D.O.
+## Square-root balancing-free methods for the frequency-weighted
+## balancing related model reduction.
+## (report in preparation)
+##
+##
+## @strong{Algorithm}@*
+## Uses SLICOT AB09ID by courtesy of
+## @uref{http://www.slicot.org, NICONET e.V.}
+## @end deftypefn
+
+## Author: Lukas Reichlin <lukas.reichlin@gmail.com>
+## Created: November 2011
+## Version: 0.1
+
+function [Gr, info] = spamodred (varargin)
+
+  [Gr, info] = __modred_ab09id__ ("spa", varargin{:});
+
+endfunction
+
+## TODO: add a test