Abstrakt
Simplified derivation of Wichmann-Kroll term is presented. The derivation uses two formulas for hypergeometric functions, but otherwise is elementary.
It is found that Laplace transform of the vacuum charge density diverges at zero momentum transfer. This divergence has nothing to do with known ultraviolet divergence.
The latter is related to the large momentum behavior of the pertinent integral, while the former to the small momentum behavior. When these divergences are removed, the energy shift caused by vacuum polarization for an ordinary hydrogen obtained here is in an exact agreement with the result obtained by Wichmann and Kroll.
Also, for muonic hydrogen the result obtained here reasonably agrees with that given in literature.