Viscosity

Through out the discussion of viscous stresses, we routinely assume

\[ \frac{\partial}{\partial x_i} \left( \frac{\partial f(x_i, x_j)}{\partial x_j} \right) = \frac{\partial}{\partial x_j} \left( \frac{\partial f(x_i, x_j)}{\partial x_i} \right) \]

which seems obvious enough. However, this is not always the case. An example of such a function can be found at this wiki.