1. E. Stiefel, M. Rössler, J. Waldvogel, C. A. Burdet, NASA Contractor Report, NASA Cr-769:Methods of Regularization for Combuting Orbits in Celestial Mechanics, 1967, Research Grant NASA NGR 52-056-001.
2. K. Stumpff,Himmelsmechanik I und II, Berlin 1959 und 1965.
3. H. J. Sperling,Computation of Keplerian Conic Sections, ARS Journal, May, 1961, p. 660.
4. R. Broucke,Regularized Special Perturbation Techniques Using Levi Civita Variables, AIAA Paper, No. 8 (1963).
5. E. Stiefel,Dynamics of Rockets and Satellites, G. V. Groves, editor, chap. IV (North-Holland Publishing Co).