1. M. J. Klein, Paul Ehrenfest, Vol. 1, (North-Holland, Amsterdam, 1970). See also: M. J. Klein, “Physics in the Making in Leiden: Paul Ehrenfest as Teacher”, Physics in the Making, A. Sarlemijn and M. J. Sparnaay, eds., (Elsevier, 1989), pp. 29-44.
2. Ref. 1, p. 189
3. P. T. Ehrenfest, “Begriffliche Grundlagen der statistischen Auffassung in der Mechanik”, Enz. d. Math. Wissensch IV 32, pp. 3–90 Teubner (1912); (b) Collected Works, pp. 213-302; (c) English translation: M. J. Moravsik, The Conceptual Foundations of the Statistical Approach in Mechanics, (Cornell University Press, Ithaca, NY 1959).
4. G. E. Uhlenbeck, “Reminiscences of Professor Paul Ehrenfest”, Am. J. Phys. 24, 431–433 (1956).
5. “ the crucial step”