@ Joe Riel: Thank you for catching those, but unfortunately: no, it won't solve my problem.

@ Robert Israel: Thank you for the reply. However, what I need is the symbolic expression, so substituting with numerical values takes me away from what I need. Furthermore, the final "Du" should in fact only have quite short expressions in elements [1,1], [2,2], and [3,3]; the rest should be zero.

I had hoped that if I could shorten the expressions before calculating "Du", then it might work. But Maple seems unable to shorten anything