In physics we often come across $$\rho=\dfrac{dq}{dV}$$
Does it mean:
$(i)$ $\displaystyle \lim_{\Delta V \to 0} \dfrac{\Delta q}{\Delta V}$
OR
$(ii)$ $\dfrac{\partial}{\partial z} \left( \dfrac{\partial}{\partial y} \left( \dfrac{\partial q}{\partial x} \right) \right)$
What does the first one mean?
The second one is a mixed partial derivative. Can it be found if $q, \dfrac{\partial q}{\partial x}, \dfrac{\partial}{\partial y} \left( \dfrac{\partial q}{\partial x} \right)$ are differentiable?