Two-photon transitions in hydrogenic atoms via a Sturmian basis for the Dirac Coulomb Green's functionC. Szymanowski, V. Véniard, R. Taïeb and A. MaquetEurophys. Lett., 37 6 (1997) 391-396DOI: https://doi.org/10.1209/epl/i1997-00161-8