1. Dennis, S. C. R., Quart. J. Mech. Appl. Math.9 (1956) 3.
2. Whittaker, E. T. and G. N. Watson, Modern Analysis, Cambridge, 1927 p. 36.
3. Jeffreys, H. and B. S. Jeffreys, Methods of Mathematical Physics, Cambridge, 1946, p. 458.
4. Dennis, S. C. R. and G. Poots, Quart. Appl. Math.14 (1956) no 3.
5. Macmillan, R. H., J. Roy. Aero. Soc.59 (1955) 772.