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A square operator combining space and time derivatives.
Also known as: Wave operator, Quabla, Box wave operator
The d'Alembertian is the wave operator of relativity: the Laplacian with a time term subtracted. It is written as a square because it is the four-dimensional counterpart of the triangular Laplacian, and sign conventions for it differ between texts.
Denotes the wave operator on spacetime, combining second derivatives in the three space directions with one in time.
Contexts: Mathematics
Differs in sign between texts according to the metric convention adopted, so its definition must be checked rather than assumed.
Contexts: Mathematics
A hollow square drawn at the height of a capital letter, set before the quantity it acts on.
The same square is the necessity operator in modal logic.