Editing D'Alembert operator (section)

==The box symbol and alternate notations==
There are a variety of notations for the d'Alembertian. The most common are the ''box'' symbol <math>\Box</math> ([[Unicode]]: {{unichar|2610|ballot box}}) whose four sides represent the four dimensions of space-time and the ''box-squared'' symbol <math>\Box^2</math> which emphasizes the scalar property through the squared term (much like the [[Laplacian]]). In keeping with the triangular notation for the [[Laplacian]], sometimes  <math>\Delta_M</math> is used.

Another way to write the d'Alembertian in flat standard coordinates is  <math>\partial^2</math>. This notation is used extensively in [[quantum field theory]], where partial derivatives are usually indexed, so the lack of an index with the squared partial derivative signals the presence of the d'Alembertian.

Sometimes the box symbol is used to represent the four-dimensional Levi-Civita [[covariant derivative]]. The symbol <math>\nabla</math> is then used to represent the space derivatives, but this is [[coordinate chart]] dependent.