--- /dev/null
+## Copyright (C) 2010 Soren Hauberg
+##
+## This program 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, or (at your option)
+## any later version.
+##
+## This program 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 this file. If not, see <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## @deftypefn {Function File} trace (@var{KP})
+## Returns the trace of the Kronecker product @var{KP}.
+##
+## If @var{KP} is a Kronecker product of two square matrices, the trace is
+## computed as the product of the trace of these two matrices. Otherwise the
+## trace is computed by forming the full matrix.
+## @seealso{@@kronprod/det, @@kronprod/rank, @@kronprod/full}
+## @end deftypefn
+
+function retval = trace (KP)
+ if (nargin != 1)
+ print_usage ();
+ endif
+
+ if (!isa (KP, "kronprod"))
+ error ("trace: input must be of class 'kronprod'");
+ endif
+
+ if (issquare (KP.A) && issquare (KP.B))
+ retval = trace (KP.A) * trace (KP.B);
+ else
+ ## XXX: Can we do something smarter here? Using 'eig' or 'svd'?
+ retval = trace (full (KP));
+ endif
+endfunction