1. S. Timoshenko, Strength of Materials, Vol. II, pp. 138?141, 3rd. ed., New York 1956.
2. R. Grammel, Z. angew. Math. Mech. 3 (1923) p. 429.
3. C. B. Biezeno and R. Grammel, Engineering Dynamics, Vol. II, pp. 213?224, London and Glasgow 1956 (translated from the 2nd German ed. 1953).
4. F. K. G. Odqvist, Ing.-Arch. 22 (1954) p. 98. A continuation of Odqvist's work will be found in a paper by A. Bjoerklund, Ing.-Arch. 23 (1955) p. 421.
5. E. R. Walters, J. Aeronaut. Sciences, New York (1945) p. 235.