Source: cirosantilli/klein-gordon-equation-in-einstein-notation
= Klein-Gordon equation in Einstein notation
The <Klein-Gordon equation> can be written in terms of the <D'alembert operator> as:
$$
\Box \psi + m^2 \psi = 0
$$
so we can expand the <D'alembert operator in Einstein notation> to:
$$
\partial_i \partial^i \psi - m^2 \psi = 0
$$