2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × FIG. 3: Feynman diagrams representing the two-loop QED corrections in the presence of an external po- tential. The notations are the same as in Fig. 1. 25