( ∂ t 2 − Δ + m 2 ) Ψ ( x _ , t ) = 0 {\displaystyle \left(\partial _{t}^{2}-\Delta +{{m}^{2}}\right)\Psi \left({\underline {x}},t\right)=0}
( ◻ + ℏ 2 c 2 m 2 ) Ψ ( x _ , t ) = 0 {\displaystyle \left(\square +{\color {Gray}{\frac {\hbar ^{2}}{c^{2}}}}m^{2}\right)\Psi \left({\underline {x}},t\right)=0}
