Strömholm C., I. Schneider, G. Sundström, L. Carata, H. Danared, S. Datz, O. Dulieu, A. Källberg, M. af Ugglas, X. Urbain, and V. Zengin, Physical Review A 52, R4320 - R4323 (1995).
Larson A., C.H. Greene, A.E. Orel, I.F. Schneider, and A. Suzor-Weiner, in (World Scientific Publishing Company, Incorporated, 2000).
and 
