This is the exercise I'm trying to solve:
Show that the general ansatz
can be transformed to the following
when u has continuous derivatives of second order
by div(u) the divergence of u is meant and when I calculate (2) and just write it down I get all the same as in (1) excepted the part with the matrix (notice: A does depend on x... is a matrix of functions):
Well so do I just have to set and the proof is complete? Not really, yeah? I did not substitute or transform anything???
What do I have to do here?