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The <laplace operator> for <Minkowski space>.

Can be nicely written with <Einstein notation> as shown at: <D'alembert operator in Einstein notation>{full}.


D'alembert operator

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


Ciro Santilli @cirosantilli 34

 Incoming links: D'alembert operator in Einstein notation