Relative errors of image approximations with the source locations. (a) Accuracy performance of Friedman, improved Friedman, Abagyan–Totrov, and Kirkwood image approximations, and multiple image approximations (M = 1 and 2), for the Poisson equation. (b) Accuracy performance of line image approximations to the exact series solution of the PB equation for λ = 1 using different asymptotic formulas, where M = 20 is used in order to discretize the line image with high accuracy to give “exact” values. Asyms. A, B, and C represent formulas (3.33), (3.35), and (3.36), respectively.