1. J. Sucher, “a3Ry Corrections to the energy levels of helium,” Columbia University Ph.D. Thesis, 1958; available from University Microfilms, Ann Arbor, Michigan.
2. J. Sucher, Phys. Rev. 109, 1010 (1958).
3. E. E. Salpeter, Phys. Rev. 87, 328 (1952).
4. In Ref. 1, eq. (1.1) was called the “no-pair Coulomb ladder equation,” in view of its connection with the so-called “ladder approximation” to the Bethe-Salpeter equation, in which only the repeated exchange of a single boson is taken into account.
5. M. Douglas and N. M. Kroll, Ann. Phys. N.Y. 82, 89 (1974).