4-gradient

[math]\nabla_i=\frac{\partial}{\partial r_i}=\partial_i[/math]

[math]\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}][/math]