Equation Icon  αβ ′ α β γδ ′ (α ′ β) ′ ′ □G γ′δ′(x,x ) + 2R γ δ (x)G γ′δ′(x,x ) = − 4πg γ′(x, x)g δ′(x, x)δ4(x,x ), (16.6 )