1. See for instance the following: Scheffers, Jahresbericht. d. deut. Math-Vereinigung, vol. 31, 1922, pp. 170–174
2. Neubauer, same journal, vol. 32, 1923, p. 310
3. Lilienthal, same journal, vol. 33, 1925, pp. 127–139
4. Ringleb, same journal, vol. 45, 1935, pp. 57–60
5. Tzitzéica, Paris Comptes Rendus, vol. 195, 1932, pp. 476–478