## 117b – Undecidability and incompleteness – Lecture 9

Let $S$ and $T$ be r.e. theories in the language of arithmetic. Assume that $S$ is strong enough to binumerate all primitive recursive relations. This suffices to prove the fixed point lemma:

For any formula $\gamma(x)$ there is a sentence $\varphi$  such that $S\vdash(\varphi\leftrightarrow\gamma(\varphi))$,

moreover, $\varphi$ can be chosen of the same complexity as $\gamma$.

The (Löb) Derivability conditions for $S,T$ are the following three statements:

• For any $\varphi$, if $T\vdash\varphi$, then $S\vdash{\rm Pr}_T(\varphi)$.
• $S\vdash\forall\varphi,\psi\,({\rm Pr}_T(\varphi\to\psi)\land {\rm Pr}_T(\varphi)\longrightarrow{\rm Pr}_T(\psi))$.
• $S\vdash\forall\varphi\,({\rm Pr}_T(\varphi)\to{\rm Pr}_T({\rm Pr}_T(\varphi)))$.

For example, $S={\sf Q}$ suffices for the proof of the fixed point lemma and of the first derivability condition and $S={\sf PA}$ suffices for the other two.

Gödel incompleteness theorems:

1. Let $S$ be r.e. and strong enough to satisfy the fixed point lemma and the first derivability condition. Let $T\vdash S$ be r.e. and consistent. Then there is a true $\Pi^0_1$ sentence $\varphi$ such that $T$ does not prove $\varphi$. If $T$ is $\Sigma^0_1$-sound, then $T$ does not prove $\lnot\varphi$ either.
2. If in addition $S$ satisfies the other two derivability conditions, then $T$ does not prove ${\rm Con}(T)$, the statement asserting the consistency of $T$.

As a corollary, ${\sf Q}$ is essentially undecidable: Not only it is undecidable, but any r.e. extension is undecidable as well. Gödel’s original statement replaced $\Sigma^0_1$-soundness with the stronger assumption of $\omega$consistency.