1. Lorentz to Einstein, December 21, 1919, in Diana Kormos Buchwald, Robert Schulmann, József Illy, Daniel J. Kennefick, Tilman Sauer, Virginia Iris Holmes, A.J. Kox, and Ze’ev Rosenkranz, ed., The Collected Papers of Albert Einstein. Vol. 9. The Berlin Years: Correspondence, January 1919-April 1920 (Princeton: Princeton University Press, 2004), Doc. 229, pp. 320-321; translated by Ann Hentschel (Princeton: Princeton University Press, 2004), Doc. 229, pp. 194-195; hereafter cited as CPAE 9.
2. Einstein to Ehrenfest, January 12, 1920; Ehrenfest to Einstein, December 9, 1919; Einstein to Lorentz, November 15, 1919, CPAE 9, Doc. 254, pp. 352-354; 212-213; Doc. 203, pp. 286-289; 173-175; Doc. 165, pp. 232-234; 138-139. For his inaugural lecture, see Albert Einstein, Äther und Relativitätstheorie. Rede gehalten am 5. Mai 1920 an der Reichs-Universität zu Leiden (Berlin: Julius Springer, 1920); reprinted in Michel Janssen, Robert Schulmann, József Illy, Christoph Lehner, Diana Kormos Buchwald, Daniel Kennefick, A.J. Kox, and David Rowe, ed., The Collected Papers of Albert Einstein. Vol. 7. The Berlin Years: Writings, 1918-1921 (Princeton: Princeton University Press, 2002), Doc. 38, pp. 306-320; G.B. Jesseryt and W. Perrett translation reprinted in Alfred Engel (Princeton: Princeton University Press, 2002), Doc. 38, pp. 160-182; hereafter cited as CPAE 7. For more on Einstein’s reassessment of Mach principle, see Michel Janssen, “Of pots and holes: Einstein’s bumpy road to general relativity,” Annalen der Physik 14 Supplement (2005), 58-85.
3. Einstein to Lorentz, January 12, 1920; Einstein to Ehrenfest, February 2, 1920, CPAE 9, Doc. 256, pp. 355-356; 214-215; Doc. 294, pp. 402-404; 246-247.
4. Willem Otterspeer, Een welbestierd budget: Honderd jaar Leids Universiteits-Fonds 1890-1990 (Leiden: Leids Universiteits-Fonds, 1990), esp. p. 54.
5. Lorentz to Einstein, February 11, 1920, CPAE 9, Doc. 308, pp. 421-422; 257-258.