Higher Order Electron-Loop Polarization Contributions

3 Higher Order Electron-Loop Polarization Contributions 

1 Wichmann-Kroll Electron-Loop Contribution of Order a(Za) m 

Contribution of the Wichmann-Kroll diagram in Fig. 3.16 with three external fields attached to the electron loop [26] may be considered in the same way as the polarization insertions in the Coulomb potential, and as we will see below it generates a correction to the Lamb shift of order a Za) m. 

A convenient representation for the Wichmann-Kroll polarization potential was obtained in [13] 

This representation allows us to calculate the correction to the Lamb shift in the same way as we have done above for the Uehling and Kallen-Sabry 

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