The diagonal lemma also requires a Gödel numbering ${\displaystyle \alpha }$ . We write ${\displaystyle \alpha (\varphi )}$  for the code assigned to ${\displaystyle \varphi }$  by the numbering. For ${\displaystyle {\overline {n}}}$ , the standard numeral of ${\displaystyle n}$  (i.e. ${\displaystyle {\overline {0}}=_{df}{\mathsf {0}}}$  and ${\displaystyle {\overline {n+1}}=_{df}{\mathsf {S}}({\overline {n}})}$ ), let ${\displaystyle \ulcorner \varphi \urcorner }$  be the standard numeral of the code of ${\displaystyle \varphi }$  (i.e. ${\displaystyle \ulcorner \varphi \urcorner }$  is ${\displaystyle {\overline {\alpha (\varphi )}}}$ ). We assume a standard Gödel numbering

Let ${\displaystyle \mathbb {N} }$  be the set of natural numbers. A first-order theory ${\displaystyle T}$  in the language of arithmetic containing ${\displaystyle {\mathsf {Q}}}$  represents the ${\displaystyle k}$ -ary recursive function ${\displaystyle f:\mathbb {N} ^{k}\rightarrow \mathbb {N} }$  if there is a formula ${\displaystyle \varphi _{f}(x_{1},\dots ,x_{k},y)}$  in the language of ${\displaystyle T}$  s.t. for all ${\displaystyle m_{1},\dots ,m_{k}\in \mathbb {N} }$ , if ${\displaystyle f(m_{1},\dots ,m_{k})=n}$  then ${\displaystyle T\vdash \forall y(\varphi _{f}({\overline {m_{1}}},\dots ,{\overline {m_{k}}},y)\leftrightarrow y={\overline {n}})}$ .

The representation theorem is provable, i.e. every recursive function is representable in ${\displaystyle T}$ .^[3]

Diagonal Lemma: Let ${\displaystyle T}$  a first-order theory containing ${\displaystyle {\mathsf {Q}}}$  (Robinson arithmetic) and let ${\displaystyle \psi (x)}$  be any formula in the language of ${\displaystyle T}$  with only ${\displaystyle x}$  as free variable. Then there is a sentence ${\displaystyle \varphi }$  in the language of ${\displaystyle T}$  s.t. ${\displaystyle T\vdash \varphi \leftrightarrow \psi (\ulcorner \varphi \urcorner )}$ .

Intuitively, ${\displaystyle \varphi }$  is a self-referential sentence which "says of itself that it has the property ${\displaystyle \psi }$ ."

Proof: Let ${\displaystyle diag_{T}:\mathbb {N} \to \mathbb {N} }$  be the recursive function which associates the code of each formula ${\displaystyle \varphi (x)}$  with only one free variable ${\displaystyle x}$  in the language of ${\displaystyle T}$  with the code of the closed formula ${\displaystyle \varphi (\ulcorner \varphi \urcorner )}$  (i.e. the substitution of ${\displaystyle \ulcorner \varphi \urcorner }$  into ${\displaystyle \varphi }$  for ${\displaystyle x}$ ) and ${\displaystyle 0}$  for other arguments. (The fact that ${\displaystyle diag_{T}}$  is recursive depends on the choice of the Gödel numbering, here the standard one.)

By the representation theorem, ${\displaystyle T}$  represents every recursive function. Thus, there is a formula ${\displaystyle \delta (x,y)}$  be the formula representing ${\displaystyle diag_{T}}$ , in particular, for each ${\displaystyle \varphi (x)}$ , ${\displaystyle T\vdash \delta (\ulcorner \varphi \urcorner ,y)\leftrightarrow y=\ulcorner \varphi (\ulcorner \varphi \urcorner )\urcorner }$ .

Let ${\displaystyle \psi (x)}$  be an arbitrary formula with only ${\displaystyle x}$  as free variable. We now define ${\displaystyle \chi (x)}$  as ${\displaystyle \exists y(\delta (x,y)\land \psi (y))}$ , and let ${\displaystyle \varphi }$  be ${\displaystyle \chi (\ulcorner \chi \urcorner )}$ . Then the following equivalences are provable in ${\displaystyle T}$ :

${\displaystyle \varphi \leftrightarrow \chi (\ulcorner \chi \urcorner )\leftrightarrow \exists y(\delta (\ulcorner \chi \urcorner ,y)\land \psi (y))\leftrightarrow \exists y(y=\ulcorner \chi (\ulcorner \chi \urcorner )\urcorner \land \psi (y))\leftrightarrow \exists y(y=\ulcorner \varphi \urcorner \land \psi (y))\leftrightarrow \psi (\ulcorner \varphi \urcorner )}$ .

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 ${\displaystyle T}$  be a first-order theory containing ${\displaystyle {\mathsf {Q}}}$  (Robinson arithmetic).

Let ${\displaystyle \psi (x,y_{1},\dots ,y_{n})}$  be any formula with free variables ${\displaystyle x,y_{1},\dots ,y_{n}}$ .

Then there is a formula ${\displaystyle \varphi (y_{1},\dots y_{n})}$  with free variables ${\displaystyle y_{1},\dots ,y_{n}}$  s.t. ${\displaystyle T\vdash \varphi (y_{1},\dots ,y_{n})\leftrightarrow \psi (\ulcorner \varphi (y_{1},\dots ,y_{n})\urcorner ,y_{1},\dots ,y_{n})}$ .

Let ${\displaystyle \psi (x,y_{1},\dots ,y_{n})}$  be any formula with free variables ${\displaystyle x,y_{1},\dots ,y_{n}}$ .

Then there is a formula ${\displaystyle \varphi (y_{1},\dots y_{n})}$  with free variables ${\displaystyle y_{1},\dots ,y_{n}}$  s.t. for all ${\displaystyle m_{1},\dots ,m_{n}\in \mathbb {N} }$ , ${\displaystyle T\vdash \varphi ({\overline {m_{1}}},\dots ,{\overline {m_{n}}})\leftrightarrow \psi (\ulcorner \varphi ({\overline {m_{1}}},\dots ,{\overline {m_{n}}})\urcorner ,{\overline {m_{1}}},\dots ,{\overline {m_{n}}})}$ .

Let ${\displaystyle \psi _{1}(x_{1},x_{2})}$  and ${\displaystyle \psi _{2}(x_{1},x_{2})}$  be formulae with free variable ${\displaystyle x_{1}}$  and ${\displaystyle x_{2}}$ .

Then there are sentence ${\displaystyle \varphi _{1}}$  and ${\displaystyle \varphi _{2}}$  s.t. ${\displaystyle T\vdash \varphi _{1}\leftrightarrow \psi _{1}(\ulcorner \varphi _{1}\urcorner ,\ulcorner \varphi _{2}\urcorner )}$  and ${\displaystyle T\vdash \varphi _{2}\leftrightarrow \psi _{2}(\ulcorner \varphi _{1}\urcorner ,\ulcorner \varphi _{2}\urcorner )}$ .

The case with ${\displaystyle n}$  many formulae is similar.

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 ${\displaystyle \psi (x)}$  with ${\displaystyle x}$  as free variable (in a sufficiently expressive language), then there exists a sentence ${\displaystyle \varphi }$  such that ${\displaystyle \varphi \leftrightarrow \psi (\ulcorner \varphi \urcorner )}$  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]

• Indirect self-reference
• List of fixed point theorems
• Self-reference
• Self-referential paradoxes

• 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.