Given the D'Alembert operator D'Alembertian $\Box$, I want to show that
$$ G(x,t,x_0,t_0):= \frac{\delta \left(t_0 + \frac{||x-x_0||}{c} -t \right)}{||x-x_0||} $$
satisfies
$$ \Box G(x,t,x_0,t_0) = - 4 \pi \delta(x-x_0)\delta(t-t_0) $$
Does anybody know if there is a nice way to see that this is true?