= D'alembert operator in Einstein notation
{c}
{title2=$\partial_i \partial^i$}
Given the function $\psi$:
$$
\psi : \R^4 \to \C
$$
the operator can be written in <Planck units> as:
$$
\partial_i \partial^i \psi(x_0, x_1, x_2, x_3) - m^2 \psi(x_0, x_1, x_2, x_3) = 0
$$
often written without function arguments as:
$$
\partial_i \partial^i \psi
$$
Note how this looks just like the <Laplacian in Einstein notation>, since the <D'alembert operator> is just a generalization of the <laplace operator> to <Minkowski space>.