1. W. Ackermann, Zum Hilbertschen Aufbau der reellen Zahlen, Math. Annalen 99(1928)118–133; English translation in [93] pp. 493–507.
2. Aristotle,Posterior Analytics, II, 3, 90 b 9 and 91 a 12; English translation inThe Works of Aristotle, 11 Vols., ed. W.D. Ross (Clarendon Press, Oxford, 1931); French translation by Jean Tricot,Les Seconds Analytiques (Vrin, 1947).
3. E.W. Beth, On Padoa's method in the theory of definition, Koningklijke Nederlandse Akademie van Wetenschappen,Proc. of the Section of Sciences, Vol. 56 (1953), series A, Mathematical Sciences, pp. 330–339; also in Indagationes Mathematicae, Vol. 15, pp. 330–339.
4. Bourbaki,Théorie des Ensembles (Hermann, 1954) EI 14–15; English translationTheory of Sets (Hermann and Addison-Wesley, 1968) p. 16.
5. D.P. Bovet and P. Crescenzi,Introduction to the Theory of Complexity (Prentice-Hall, 1994) p. XI + 282.