$\frac{d}{dx}$ $\frac{\delta}{\delta x}$ $\frac{\Delta}{\Delta x}$ $\frac{\partial}{\partial x}$
$\frac{dx}{dy}$ $\frac{\delta x}{\delta y}$ $\frac{\Delta x}{\Delta y}$ $\frac{\partial x}{\partial y}$
$dx$ $\delta x$ $\Delta x$ $\partial x$
$\nabla a$ $\nabla^2 a$ $\overrightarrow{\nabla . a}$