1. Pollard, H., Mathematical Introduction to Celestial Mechanics. New York: Prentice-Hall 1966.
2. Pollard, H., Proc. Amer. Math. Soc. 18, 399?401 (1967).
3. Siegel, C. L., Ann. Math. 42, 127?168 (1941).
4. Von Ziepel, H., Ark. Mat. Astr. Fys. 4, No. 32 (1908).
5. Wintner, A., The Analytical Foundations of Celestial Mechanics. Princeton University Press 1941.