cantors-attic

Climb into Cantor’s Attic, where you will find infinities large and small. We aim to provide a comprehensive resource of information about all notions of mathematical infinity.

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Cantor's Attic (original site)
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Stable

Stability was developed as a large countable ordinal property in order to try to generalize the different strengthened variants of admissibility. More specifically, they capture the various assertions that LαKP+A for different axioms A by saying that LαKP+A for many axioms A. One could also argue that stability is a weakening of Σ1-correctness (which is trivial) to a nontrivial form.

Definition and Variants

Stability is defined using a reflection principle. A countable ordinal α is called stable iff LαΣ1L; equivalently, LαΣ1Lω1. (Madore, 2017)

Variants

There are quite a few (weakened) variants of stability:(Madore, 2017)

Properties

Any L-stable ordinal is stable. This is because LαL=Lα and LL=L. (Jech, 2003) Any L-countable stable ordinal is L-stable for the same reason. Therefore, an ordinal is L-stable iff it is L-countable and stable. This property is the same for all variants of stability.

The smallest stable ordinal is also the smallest ordinal α such that LαKP+Σ21-reflection, which in turn is the smallest ordinal which is not the order-type of any Δ21-ordering of the natural numbers. The smallest stable ordinal σ has the property that any Σ1(Lσ) subset of ω is ω-finite. (Madore, 2017)

If there is an ordinal η such that LηZFC (i.e. the minimal height of a transitive model of ZFC) then it is smaller than the least stable ordinal. On the other hand, the sizes of the least (+1)-stable ordinal and the least nonprojectible ordinal lie between the least recursively weakly compact and the least Σ2-admissible (the same for other weakened variants of stability defined above). (Madore, 2017)

References

  1. Madore, D. (2017). A zoo of ordinals. http://www.madore.org/ david/math/ordinal-zoo.pdf
  2. Jech, T. J. (2003). Set Theory (Third). Springer-Verlag. https://logic.wikischolars.columbia.edu/file/view/Jech%2C+T.+J.+%282003%29.+Set+Theory+%28The+3rd+millennium+ed.%29.pdf
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