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Deleted "that $Q*$ has the thesis" and "the thesis".
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Frode Alfson Bjørdal
Frode Alfson Bjørdal
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It is an odd and arguably unacceptable situation that $PA$ does not have the thesis   $\vdash_{PA}(Pr_{PA}\ulcorner A\urcorner\to A)$ for false recursive sentences $A$.

However, it is not clear to me that Löb's theorem is already derivable in Robinson arithmetic $Q$, for one cannot assume that the provability predicate of $Q$ obeys all the Löb derivability conditions. (Compare to these matter question A question on the provability predicate of Q).

Are there natural omega consistent extensions $Q*$ of $Q$ such that $Q*$ has the thesis   $\vdash_{Q*}(Pr_{Q*}\ulcorner A\urcorner\to A)$ for all $\Delta_1$ sentences?

It is an odd and arguably unacceptable situation that $PA$ does not have the thesis $\vdash_{PA}(Pr_{PA}\ulcorner A\urcorner\to A)$ for false recursive sentences $A$.

However, it is not clear to me that Löb's theorem is already derivable in Robinson arithmetic $Q$, for one cannot assume that the provability predicate of $Q$ obeys all the Löb derivability conditions. (Compare to these matter question A question on the provability predicate of Q).

Are there natural omega consistent extensions $Q*$ of $Q$ such that $Q*$ has the thesis $\vdash_{Q*}(Pr_{Q*}\ulcorner A\urcorner\to A)$ for all $\Delta_1$ sentences?

It is an odd and arguably unacceptable situation that $PA$ does not have   $\vdash_{PA}(Pr_{PA}\ulcorner A\urcorner\to A)$ for false recursive sentences $A$.

However, it is not clear to me that Löb's theorem is already derivable in Robinson arithmetic $Q$, for one cannot assume that the provability predicate of $Q$ obeys all the Löb derivability conditions. (Compare to these matter question A question on the provability predicate of Q).

Are there natural omega consistent extensions $Q*$ of $Q$ such   $\vdash_{Q*}(Pr_{Q*}\ulcorner A\urcorner\to A)$ for all $\Delta_1$ sentences?

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Frode Alfson Bjørdal
Frode Alfson Bjørdal
  • 3.6k
  • 1
  • 15
  • 35

Can extensions of $Q$ contradict Löb with recursive reflection?

It is an odd and arguably unacceptable situation that $PA$ does not have the thesis $\vdash_{PA}(Pr_{PA}\ulcorner A\urcorner\to A)$ for false recursive sentences $A$.

However, it is not clear to me that Löb's theorem is already derivable in Robinson arithmetic $Q$, for one cannot assume that the provability predicate of $Q$ obeys all the Löb derivability conditions. (Compare to these matter question A question on the provability predicate of Q).

Are there natural omega consistent extensions $Q*$ of $Q$ such that $Q*$ has the thesis $\vdash_{Q*}(Pr_{Q*}\ulcorner A\urcorner\to A)$ for all $\Delta_1$ sentences?