Is is written ◻ {\displaystyle \Box } or ◻ 2 {\displaystyle \Box ^{2}} ?
Is it equal to ∇ 2 − 1 c 2 ∂ 2 ∂ t 2 {\displaystyle \nabla ^{2}-{\frac {1}{c^{2}}}{\frac {\partial ^{2}}{\partial t^{2}}}} or 1 c 2 ∂ 2 ∂ t 2 − ∇ 2 {\displaystyle {\frac {1}{c^{2}}}{\frac {\partial ^{2}}{\partial t^{2}}}-\nabla ^{2}} ? Sources differ again.
