Editing D'Alembert operator (section)

==Green's function==
The [[Green's function]], <math>G\left(\tilde{x} - \tilde{x}'\right)</math>, for the d'Alembertian is defined by the equation 
:<math> \Box G\left(\tilde{x} - \tilde{x}'\right) = \delta\left(\tilde{x} - \tilde{x}'\right)</math>

where <math>\delta\left(\tilde{x} - \tilde{x}'\right)</math> is the multidimensional [[Dirac delta function]] and <math>\tilde{x}</math> and  <math>\tilde{x}'</math> are two points in Minkowski space.

A special solution is given by the ''retarded Green's function'' which corresponds to signal [[propagator|propagation]] only forward in time<ref>{{cite web|author=S. Siklos|title=The causal Green's function for the wave equation|url=http://www.damtp.cam.ac.uk/user/stcs/courses/fcm/handouts/wave_equation.pdf|access-date=2 January 2013|archive-date=30 November 2016|archive-url=https://web.archive.org/web/20161130174612/http://www.damtp.cam.ac.uk/user/stcs/courses/fcm/handouts/wave_equation.pdf|url-status=dead}}</ref>
:<math>G\left(\vec{r}, t\right) = \frac{1}{4\pi r} \Theta(t) \delta\left(t - \frac{r}{c}\right)</math>

where <math>\Theta</math> is the [[Heaviside step function]].