1. “Die Widerspruchsfreiheit der allgemeinen Mengenlehre”;Ackermann;Mathematische Annalen,1937
2. Richard Dedekind, Was sind und was sollen die Zahlen?, Braunschweig, 1888. English translation by W.W. Beman as “The nature and meaning of numbers”, in Richard Dedekind, Essays on the Theory of Numbers, Open Court, 1901, reprinted by Dover, New York, 1963, pp. 31-115
3. letter to Keferstein;Dedekind,1890
4. “A more perspicuous formal system for predicativity”;Feferman,1979
5. Gottlob Frege, Die Grundlagen der Arithmetik: Eine logisch mathematische Untersuchung über den Begriff der Zahl, Wilhelm Koebner, Breslau, 1884, 119pp. English translation by J.L. Austin, Basil Blackwell, Oxford, 1950