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:

$□ψ+m_{2}ψ=0$

$∂_{i}∂_{i}ψ−m_{2}ψ=0$

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:

$□ψ+m_{2}ψ=0$

$∂_{i}∂_{i}ψ−m_{2}ψ=0$