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My question is from chapter #21 "Newton's theory as a First Approximation", from the expression (68a). It is the equation for gravitational potential. He says that we can get (68a) by (67) and (68),I tried but can't figure it out. Particularly, where is $r$ and $8\pi$ comes from?
It come from the Green function $$G({\bf r},0)=1/4\pi |{\bf r}| $$ of the of the Laplace equation in 68, and the fact that eq 67 shows that $g_{44}$ is one half the usual gravitational potential.
