left_mtimes.m

curves ti si s a nice code.

function n = left_mtimes(k, x)
%  Helper function for mtimes.m
%
%  if x is a scalar, use the property that
%  x * kron(A, B) = kron(x * A, B)
%
%  if x is a vector such that x = vec(X),
%  use the property that 
%  kron(A, B) * x = vec(B * X * A')
%

kasize = size(k.a);
kbsize = size(k.b);
xsize = size(x);

if (length(x) == 1)
   n = kronMatrix(k.a * x, k.b);
elseif xsize(1) == kasize(2)*kbsize(2)
   try
      n = zeros(kasize(1) * kbsize(1), xsize(2));
      for i = 1 : xsize(2)
         tmp = reshape(x(:,i), kbsize(2), kasize(2));
         tmp = k.b * tmp * k.a';
         n(:,i) = reshape(tmp, kbsize(1) * kasize(1),1);
      end
   catch
      error(lasterr)
   end
elseif ( kbsize(2) == xsize(1) & xsize(2) == kasize(2) )
   n = k.b * x * k.a';
end