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+## Copyright (C) 2012   Lukas F. Reichlin
+## Copyright (C) 2012   Megan Zagrobelny
+##
+## 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{l}, @var{p}, @var{e}] =} lqe (@var{sys}, @var{q}, @var{r})
+## @deftypefnx {Function File} {[@var{l}, @var{p}, @var{e}] =} lqe (@var{sys}, @var{q}, @var{r}, @var{s})
+## @deftypefnx {Function File} {[@var{l}, @var{p}, @var{e}] =} lqe (@var{a}, @var{g}, @var{c}, @var{q}, @var{r})
+## @deftypefnx {Function File} {[@var{l}, @var{p}, @var{e}] =} lqe (@var{a}, @var{g}, @var{c}, @var{q}, @var{r}, @var{s})
+## @deftypefnx {Function File} {[@var{l}, @var{p}, @var{e}] =} lqe (@var{a}, @var{[]}, @var{c}, @var{q}, @var{r})
+## @deftypefnx {Function File} {[@var{l}, @var{p}, @var{e}] =} lqe (@var{a}, @var{[]}, @var{c}, @var{q}, @var{r}, @var{s})
+## Kalman filter for continuous-time systems.
+##
+## @example
+## @group
+## .
+## x = Ax + Bu + Gw   (State equation)
+## y = Cx + Du + v    (Measurement Equation)
+## E(w) = 0, E(v) = 0, cov(w) = Q, cov(v) = R, cov(w,v) = S
+## @end group
+## @end example
+##
+## @strong{Inputs}
+## @table @var
+## @item sys
+## Continuous or discrete-time LTI model (p-by-m, n states).
+## @item a
+## State transition matrix of continuous-time system (n-by-n).
+## @item g
+## Process noise matrix of continuous-time system (n-by-g).
+## If @var{g} is empty @code{[]}, an identity matrix is assumed.
+## @item c
+## Measurement matrix of continuous-time system (p-by-n).
+## @item q
+## Process noise covariance matrix (g-by-g).
+## @item r
+## Measurement noise covariance matrix (p-by-p).
+## @item s
+## Optional cross term covariance matrix (g-by-p), s = cov(w,v).
+## If @var{s} is empty @code{[]} or not specified, a zero matrix is assumed.
+## @end table
+##
+## @strong{Outputs}
+## @table @var
+## @item l
+## Kalman filter gain matrix (n-by-p).
+## @item p
+## Unique stabilizing solution of the continuous-time Riccati equation (n-by-n).
+## Symmetric matrix.  If @var{sys} is a discrete-time model, the solution of the
+## corresponding discrete-time Riccati equation is returned.
+## @item e
+## Closed-loop poles (n-by-1).
+## @end table
+##
+## @strong{Equations}
+## @example
+## @group
+## .
+## x = Ax + Bu + L(y - Cx -Du)          
+##
+## E = eig(A - L*C)
+## 
+## @end group
+## @end example
+## @seealso{dare, care, dlqr, lqr, dlqe}
+## @end deftypefn
+
+## Author: Lukas Reichlin <lukas.reichlin@gmail.com>
+## Created: April 2012
+## Version: 0.1
+
+function [l, p, e] = lqe (a, g, c, q = [], r = [], s = [])
+
+  if (nargin < 3 || nargin > 6)
+    print_usage ();
+  endif
+
+  if (isa (a, "lti"))
+    [l, p, e] = lqr (a.', g, c, q);         # lqe (sys, q, r, s), g=I, works like  lqr (sys.', q, r, s).'
+  elseif (isempty (g))
+    [l, p, e] = lqr (a.', c.', q, r, s);    # lqe (a, [], c, q, r, s), g=I, works like  lqr (a.', c.', q, r, s).'
+  elseif (columns (g) != rows (q) || ! issquare (q))
+    error ("lqe: matrices g(%dx%d) and q(%dx%d) have incompatible dimensions", \
+            rows (g), columns (g), rows (q), columns (q));
+  elseif (isempty (s))
+    [l, p, e] = lqr (a.', c.', g*q*g.', r);
+  elseif (columns (g) != rows (s))
+    error ("lqe: matrices g(%dx%d) and s(%dx%d) have incompatible dimensions", \
+            rows (g), columns (g), rows (s), columns (s));
+  else
+    [l, p, e] = lqr (a.', c.', g*q*g.', r, g*s);
+  endif
+
+  l = l.';
+  
+  ## NOTE: for discrete-time sys, the solution L' from DARE
+  ##       is different to L from DLQE (a, s)
+
+endfunction