% -*- texinfo -*-
% @deftypefn {Function File} {} commutation_matrix (@var{m}, @var{n})
% Return the commutation matrix
% @tex
% $K_{m,n}$
% @end tex
% @ifnottex
% K(m,n)
% @end ifnottex
% which is the unique
% @tex
% $m n \times m n$
% @end tex
% @ifnottex
% @var{m}*@var{n} by @var{m}*@var{n}
% @end ifnottex
% matrix such that
% @tex
% $K_{m,n} \cdot {\rm vec} (A) = {\rm vec} (A^T)$
% @end tex
% @ifnottex
% @math{K(m,n) * vec(A) = vec(A')}
% @end ifnottex
% for all
% @tex
% $m\times n$
% @end tex
% @ifnottex
% @math{m} by @math{n}
% @end ifnottex
% matrices
% @tex
% $A$.
% @end tex
% @ifnottex
% @math{A}.
% @end ifnottex
%
% If only one argument @var{m} is given,
% @tex
% $K_{m,m}$
% @end tex
% @ifnottex
% @math{K(m,m)}
% @end ifnottex
% is returned.
%
% See Magnus and Neudecker (1988), Matrix differential calculus with
% applications in statistics and econometrics.
% @end deftypefn