Diagonal lemma
In mathematical logic, the diagonal lemma (also known as diagonalization lemma, self-reference lemma or fixed point theorem) establishes the existence of self-referential sentences in certain formal theories.
A particular instance of the diagonal lemma was used by Kurt Gödel in 1931 to construct his proof of the incompleteness theorems as well as in 1933 by Tarski to prove his undefinability theorem. In 1934, Carnap was the first to publish the diagonal lemma at some level of generality.[1] The diagonal lemma is named in reference to Cantor's diagonal argument in set and number theory.
The diagonal lemma applies to any sufficiently strong theories capable of representing the diagonal function. Such theories include first-order Peano arithmetic , the weaker Robinson arithmetic as well as any theory containing (i.e. which interprets it).[2] A common statement of the lemma (as given below) makes the stronger assumption that the theory can represent all recursive functions, but all the theories mentioned have that capacity, as well.
Background
[edit]Gödel Numbering
[edit]The diagonal lemma also requires a Gödel numbering . We write for the code assigned to by the numbering. For , the standard numeral of (i.e. and ), let be the standard numeral of the code of (i.e. is ). We assume a standard Gödel numbering
Representation Theorem
[edit]Let be the set of natural numbers. A first-order theory in the language of arithmetic containing represents the -ary recursive function if there is a formula in the language of s.t. for all , if then .
The representation theorem is provable, i.e. every recursive function is representable in .[3]
The Diagonal Lemma and its proof
[edit]Diagonal Lemma: Let a first-order theory containing (Robinson arithmetic) and let be any formula in the language of with only as free variable. Then there is a sentence in the language of s.t. .
Intuitively, is a self-referential sentence which "says of itself that it has the property ."
Proof: Let be the recursive function which associates the code of each formula with only one free variable in the language of with the code of the closed formula (i.e. the substitution of into for ) and for other arguments. (The fact that is recursive depends on the choice of the Gödel numbering, here the standard one.)
By the representation theorem, represents every recursive function. Thus, there is a formula be the formula representing , in particular, for each , .
Let be an arbitrary formula with only as free variable. We now define as , and let be . Then the following equivalences are provable in :
.
Some Generalizations
[edit]There are various generalizations of the Diagonal Lemma. We present only three of them; in particular, combinations of the below generalizations yield new generalizations.[4] Let be a first-order theory containing (Robinson arithmetic).
Diagonal Lemma with Parameters
[edit]Let be any formula with free variables .
Then there is a formula with free variables s.t. .
Uniform Diagonal Lemma
[edit]Let be any formula with free variables .
Then there is a formula with free variables s.t. for all , .
Simultaneous Diagonal Lemma
[edit]Let and be formulae with free variable and .
Then there are sentence and s.t. and .
The case with many formulae is similar.
History
[edit]The lemma is called "diagonal" because it bears some resemblance to Cantor's diagonal argument.[5] The terms "diagonal lemma" or "fixed point" do not appear in Kurt Gödel's 1931 article or in Alfred Tarski's 1936 article.
In 1934, Rudolf Carnap was the first to publish the diagonal lemma in some level of generality, which says that for any formula with as free variable (in a sufficiently expressive language), then there exists a sentence such that is true (in some standard model).[6] Carnap's work was phrased in terms of truth rather than provability (i.e. semantically rather than syntactically).[7] Remark also that the concept of recursive functions was not yet developed in 1934.
The diagonal lemma is closely related to Kleene's recursion theorem in computability theory, and their respective proofs are similar.[8] In 1952, Léon Henkin asked whether sentences that state their own provability are provable. His question led to more general analyses of the diagonal lemma, especially with Löb's theorem and provability logic.[9]
See also
[edit]Notes
[edit]- ^ See Smoryński 2022, Sec. 3.
- ^ See Hájek and Pudlák 2016, Chap. III.
- ^ See Hinman 2005, Chap 4.6 for additional details and a proof of this theorem.
- ^ See Smoryński 2022, Sec. 3 or Hájek and Pudlák 2016, III.2.a
- ^ See, for example, Gaifman (2006).
- ^ See Carnap, 1934, and Gödel, 1986, p. 363, fn 23.
- ^ See Smoryński 2022, Sec. 3.
- ^ See Gaifman, 2006 or Smoryński 2022, Sec. 3.
- ^ See Smoryński 2022, Sec. 3.
References
[edit]- George Boolos and Richard Jeffrey, 1989. Computability and Logic, 3rd ed. Cambridge University Press. ISBN 0-521-38026-X ISBN 0-521-38923-2
- Rudolf Carnap, 1934. Logische Syntax der Sprache. (English translation: 2003. The Logical Syntax of Language. Open Court Publishing.)
- Haim Gaifman, 2006. 'Naming and Diagonalization: From Cantor to Gödel to Kleene'. Logic Journal of the IGPL, 14: 709–728.
- Petr Hájek & Pavel Pudlák, 2016 (first edition 1998). Metamathematics of First-Order Arithmetic. Springer Verlag.
- Peter Hinman, 2005. Fundamentals of Mathematical Logic. A K Peters. ISBN 1-56881-262-0
- Mendelson, Elliott, 1997. Introduction to Mathematical Logic, 4th ed. Chapman & Hall.
- Panu Raatikainen, 2015a. The Diagonalization Lemma. In Stanford Encyclopedia of Philosophy, ed. Zalta.
- Panu Raatikainen, 2015b. Gödel's Incompleteness Theorems. In Stanford Encyclopedia of Philosophy, ed. Zalta.
- Raymond Smullyan, 1991. Gödel's Incompleteness Theorems. Oxford Univ. Press.
- Raymond Smullyan, 1994. Diagonalization and Self-Reference. Oxford Univ. Press.
- Craig Smoryński, 2023. 'The early history of formal diagonalization'. Logic Journal of the IGPL, 31.6: 1203–1224.
- Alfred Tarski (1936). "Der Wahrheitsbegriff in den formalisierten Sprachen" (PDF). Studia Philosophica. 1: 261–405. Archived from the original (PDF) on 9 January 2014. Retrieved 26 June 2013.
- Alfred Tarski, tr. J. H. Woodger, 1983. 'The Concept of Truth in Formalized Languages'. English translation of Tarski's 1936 article. In A. Tarski, ed. J. Corcoran, 1983, Logic, Semantics, Metamathematics, Hackett.