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Jensen claimed that for any finite increasing sequence countable admissible ordinals $\omega= \alpha_0<\alpha_1\cdots <\alpha_n$, there is a real $x$ so that, for each $m\leq n$, $\alpha_m$ is the $m+1$-th admissible ordinal relative to $x$.

Anybody knows the proof? Or where to find it?

flag	asked Apr 5 2012 at 2:58 Liang Yu 670●5●8

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mathematik.hu-berlin.de/~raesch/org/jensen.html (The paper titled "Admissible sets") – Andres Caicedo Apr 5 2012 at 3:05

Thanks. It's really painful to read the handwritten manuscripts... – Liang Yu Apr 5 2012 at 3:32

(I just emailed you a typeset version of the notes.) – Andres Caicedo Apr 5 2012 at 6:27

Hello Andres. Can you email me typset version of the notes too? hollowdead1@gmail.com – Math-player Aug 25 at 13:46

Andres me too? gerdes@invariant.org – Peter Gerdes Jan 12 at 11:40

Theodore Slaman · Accepted Answer · 2013-02-03 19:57:50Z UTC

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There is a model theoretic proof of the generalization to countable sequences which appears in a paper by Simpson and Weitkamp.

High and low Kleene degrees of coanalytic sets Stephen G. Simpson & Galen Weitkamp Journal of Symbolic Logic 48 (2):356-368 (1983)

I believe that this proof is due to Harrington, based on Jensen's and Friedman's model theoretic proof of Sacks's theorem that every countable admissible ordinal is the least admissible relative to some real.

answered Feb 3 at 19:57

Theodore Slaman
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Hi Ted, nice to see you here! – Andres Caicedo Feb 3 at 20:31

Yes, Ted, welcome to MathOverflow. – Joel David Hamkins Feb 3 at 21:11

Ted, thanks. In Simpon-Weitkamp's paper, it is claimed that Harrington has a model theoretical proof. But where to find it? – Liang Yu Feb 4 at 4:14

I asked Prof. Jensen, when he was in NUS, whether he has a model theoretical proof, or by applying Barwise compactness, of his result. He said no. So I guess Harrington's proof must be highly nontrivial. – Liang Yu Feb 4 at 7:14

Yu, the result is Theorem 4.1 of the Simpson and Weitkamp paper and they include a proof. The proof comes before the attribution to Harrington. – Theodore Slaman Feb 5 at 2:48

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Countable admissible ordinals

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