d'Alembertian Operator

Written in the Notation of Partial Derivatives,

$$\vbox{\hrule height.6pt\hbox{\vrule width.6pt height6pt \ker... ... \equiv \nabla^2 - {1\over c^2} {\partial^2\over\partial t^2},$$

where $c$ is the speed of light. Writing in Tensor notation,

$$\vbox{\hrule height.6pt\hbox{\vrule width.6pt height6pt \ker... ...ppa} - \Gamma^\lambda {\partial \phi\over \partial x^\lambda}.$$

See also Harmonic Coordinates