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Let there be an omega sequence of ordinals such that the first is the least $\Sigma_1$-admissible ordinal and the $n+1$st is the least $\Sigma_{n+1}$-admissible ordinal. What is the name, if any, of the union of all of these? Incidentally, $L$ at the level of this ordinal would be the minimal model of ZFC minus the power set axiom.

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I don't see why the last sentence is true. – François G. Dorais Jul 21 '13 at 21:49
up vote 4 down vote accepted

This ordinal is not as large as you claim. In particular, the constructible universe up to this ordinal does not satisfy ZFC minus power set, and the ordinal is not even admissible.

To see this, let $\gamma_n$ be the least $\Sigma_n$-admissible ordinal (which I understand you to mean that $L_{\gamma_n}$ satisfies KP plus $\Sigma_n$-replacement, or equivalently, $\Sigma_n$-collection); and then the ordinal in question is $\gamma=\sup_n\gamma_n$ and the corresponding structure $L_\gamma$. But this structure does not satisfy KP, since the map $n\mapsto\gamma_n$ on domain $n\in \omega$ is $\Sigma_1$-definable in $L_\gamma$. The point is that whether some $L_\beta$ satisfies a certain sentence is a $\Delta_0$ espressible assertion about $L_\beta$, since all quantifiers are bounded by $L_\beta$, and whether $\beta=\gamma_n$ is a $\Sigma_1$-property of $\beta$ inside $L_\gamma$. Namely, $\beta=\gamma_n$ if and only if there is a transitive set $X$ that thinks it is $L_\beta$ and there is a satisfaction predicate on this set fulfilling Tarski's recursive definition of truth and according to this satisfaction predicate, the set $X$ satisfies KP and every instance of $\Sigma_n$-collection, and finally no smaller $\beta'\lt\beta$ satisfies all those.

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It seems that one can get it down to $\Delta_1$, since the structure $L_\beta$ is unique and so is its satisfaction predicate. So the relation $\beta=\gamma_n$ is a $\Delta_1$ property of $\beta$ and $n$ inside $L_\gamma$. – Joel David Hamkins Jul 21 '13 at 22:06
Would the least admissible ordinal above be at the level where ZFC minus power set has a model? – Frode Bjørdal Jul 21 '13 at 22:26
@FrodeBjørdal Unless I'm very confused, the next admissible ordinal strictly above any given $\alpha$ is never $\Sigma_2$-admissible. – Andreas Blass Jul 21 '13 at 22:55
Let S be a set that does not contain zero such that S is not definable by ZFC minus the power set, and let f be a function from N onto S. Is the union of the sets in the omega sequence s(0)=the f(0)'th Sigma (0)-admissible ordinal and s(n+1)=the f(n+1)'th Sigma(n+1) admissible ordinal and ordinal where L models ZFC minus power set? – Frode Bjørdal Jul 21 '13 at 23:25
I would expect that the answer is negative. Note that your $f$ need not grow very rapidly. If $A$ is any set of natural numbers that isn't definable in ZFC minus power set, then you could take $S$ to be $\{2n:n\in A\}\cup\{2n+1:n\notin A\}$. – Andreas Blass Jul 22 '13 at 23:00

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