4-gradient

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

[math]\mathbf \partial_\mu \equiv \Bigl [ \frac{\partial}{\partial t}\quad \frac{\partial}{\partial x}\quad \frac{\partial}{\partial y}\quad \frac{\partial}{\partial z}]=[\frac{\partial}{\partial x}\quad \frac{\partial}{\partial x}\quad \frac{\partial}{\partial x}\quad \frac{\partial}{\partial x}][/math]