4-gradient

$\nabla_i=\frac{\partial}{\partial r_i}=\partial_i$

$\mathbf \partial_\mu \equiv[\frac{\partial}{\partial t}\ \frac{\partial}{\partial x}\ \frac{\partial}{\partial y}\ \frac{\partial}{\partial z}]=[\frac{\partial}{\partial x}\ \frac{\partial}{\partial x}\ \frac{\partial}{\partial x}\ \frac{\partial}{\partial x}]$