1. An account of them is given in Whittaker and Watson, A Course of Modern Analysis, Chap. 19 (1920). This is, however, quite incomplete now because of the many more recent investigations by British and Scottish mathematicians.
2. See e.g. Goldstein, Trans. Cambr. Phil. Soc. 23, 303, (1927) Par. 1.5. This memoir contains a good many of the newer results not given in Whittaker and Watson.
3. Jeffreys, Proc. London Math. Soc. 23, 437, (1924–25). This paper and the one preceding it are especially interesting in that the methods of approximate integration which he uses are closely related to those by means of which the connection between classical mechanics and quantum mechanics is established.
4. Hund, Zeits. f. Phys. 40, 742, (1927) especially footnote, p. 750.
5. See e.g., Hund, Ince, Jl. London Math. Soc. 2, 46, (1927).