# Issue with Differentiation over a tensor

I am new user in Mathematica. I am working with liquid crystal theory, and one of the notation there is the tensor:

$\Pi_{ij} = \frac {\partial {F_{el}}} { \partial ( \partial {n_i} / \partial { x_j})}$

Where numerator is scalar, denominator is tensor (matrix).

In order to calculate it in Mathematica, I introduce:

r={x,y,z}
nd[x_, y_, z_] = {n1[x, y], n2[x, y], 0}
Fel = ***Some expression (nd) ***
g = D[nd[x, y, z], {r}]


After that I need to take a derivative of Fel over a tensor g. But as long as I am restricted to a 2D space {x,y}, my tensor g looks like:

g= (n1^(1,0)(x,y)   n1^(0,1)(x,y)   0
n2^(1,0)(x,y)   n2^(0,1)(x,y)   0
0               0               0)


It is not allowed to differentiate over zero. Mathematically, zero values must be omitted (the corresponding term must be set to zero).

Thanks,

You can fill zeros of the tensor with variables that are not used, like z1,z2,z3,.... Now the derivative over this variables is zero, so I got desired result