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Wichmann-Kroll potentials, can be found, e.g., in Ref. [56]. The contributions of these diagrams are given by 

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The diagrams of type C (so-called ’magnetic-loop’ diagrams) involve the matrix elements of the magnetic-field-induced vacuum-polarization potential U ml $_{VP}$. It also requires the charge renormal- ization to make it finite. Our present treatment of this poten tial is restricted to the Uehling (free- electron-loop) approximation. The expression for this potential in the Uehling approximation in case of the hyperfine interaction can be found in Ref. [57] forthe point-dipole nuclear model, and in Ref. [47] for the sphere model of the nuclear magnetization distribution. In case of the g factor this potential in the Uehling approximation is equal to zero. The contribution of the diagrams of type C is given by 

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The diagram of type F contains the interelectronic-interaction operator modifi ed by the vacuum polarization I$_{VP}$ ( ω ) . Our present treatment of this term is restricted to the Uehling approximation. 

