Klein-Gordon equation in Einstein notation (source code)

= 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:
$$
\partial_i \partial^i \psi - m^2 \psi = 0
$$