= Einstein notation for partial derivatives
The <Einstein summation convention> works will with <partial derivatives> and it is widely used in <particle physics>.
In particular, the <divergence> and the <Laplacian> can be succintly expressed in this notation:
* <Divergence in Einstein notation>{full}
* <Laplacian in Einstein notation>{full}
In order to expresse partial derivatives, we must use what <Ciro Santilli> calls the "<partial index partial derivative notation>", which refers to variales with indices such as $x_0$, $x_1$, $x_2$, $\partial_0$, $\partial_1$ and $\partial_2$ instead of the usual letters $x$, $y$ and $z$.