Given the function $ψ$:
the operator can be written in Planck units as:
often written without function arguments as:
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.

$ψ:R_{4}→C$

$∂_{i}∂_{i}ψ(x_{0},x_{1},x_{2},x_{3})−m_{2}ψ(x_{0},x_{1},x_{2},x_{3})=0$

$∂_{i}∂_{i}ψ$

The Klein-Gordon equation can be written in terms of the D'alembert operator as:
so we can expand the D'alembert operator in Einstein notation to:

$□ψ+m_{2}ψ=0$

$∂_{i}∂_{i}ψ−m_{2}ψ=0$