Is There a Simple Formula for the Green Function of the Klein-Gordon Equation?

[paweld]

There exists very simple formula for Green function for wave equation:
$$G(t,x,t',x') = \delta (t-t'\pm \frac{|x-x'|}{c})/|x-x'|$$.
I wonder whether there exist similar formula for Green function
for Klein-Gordon equation (with mass >0) for any boundary condition.

 

[madness]

I think it's just the Feynman propagator (http://en.wikipedia.org/wiki/Propagator#Feynman_propagator)