Editing Propagator (section)

===Green's functions for the Klein–Gordon equation===
The retarded, advanced and Feynman propagators defined above are all Green's functions for the Klein–Gordon equation.

They are related to the singular functions by<ref name="BD"/>
:<math>G_\text{ret}(x-y) = \Delta(x-y) \Theta(x^0-y^0) </math>
:<math>G_\text{adv}(x-y) = -\Delta(x-y) \Theta(y^0-x^0) </math>
:<math>2 G_F(x-y) = -i \,\Delta_1(x-y) + \varepsilon(x^0 - y^0) \,\Delta(x-y) </math>
where <math>\varepsilon(x^0-y^0)</math> is the sign of <math>x^0-y^0</math>.