Equation Icon  1 ∫ ( ′ ′′ ′′ ′ ) γαβ(x) = − --- G +αβγ′δ′(x,x′)∇ 𝜖γγ δ(x′) − γ γδ (x ′)∇ 𝜖G +αβγ′δ′(x,x′) dΣ 𝜖′, (16.20 ) 4π Σ