Jentschura, Ulrich David Division Prof. Dr. Christoph H. Keitel, MPI for Nuclear Physics, Max Planck Society;
http://link.aps.org/doi/10.1103/PhysRevA.70.052108 (Publisher version)
Jentschura, U. D. (2004). Two-Loop Bethe logarithms for higher excited S levels. Physical Review A, 70(5): 052108. doi:10.1103/PhysRevA.70.052108.