Equation Icon  lim 𝜖δ± (σ + 𝜖) = 0, (13.6 ) 𝜖→0+ lim 𝜖δ′± (σ + 𝜖) = 0, (13.7 ) 𝜖→0+ lim+𝜖δ′±′(σ + 𝜖) = 2π δ4(x,x′) (13.8 ) 𝜖→0