see at _[logic - Mathematical logic](logic#MathematicalLogic)_. and at *[[formal logic]]* ## References * {#HilbertAckermann59} [[David Hilbert]], [[Wilhelm Ackermann]], _Grundzüge der Theoretischen Logik_ , 4th ed. Springer Heidelberg 1959 [1928] * [[Georg Kreisel]], *Mathematical Logic*, in T. Saaty et al. (ed.), *Lectures on Modern Mathematics III*, Wiley New York (1965) 95-195 * {#BellMach77} [[John Lane Bell]], M. Machover, _A Course in Mathematical Logic_, North-Holland Amsterdam 1977. (ch. 10,§5) ([ISBN:9780720428445](https://www.elsevier.com/books/a-course-in-mathematical-logic/bell/978-0-7204-2844-5)) category: logic