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
for different axioms by saying that
for many axioms . One could also
argue that stability is a weakening of
-correctness
(which is trivial) to a nontrivial form.
Definition and Variants
Stability is defined using a reflection principle. A countable ordinal
is called stable iff ;
equivalently, .
(Madore, 2017)
Variants
There are quite a few (weakened) variants of
stability:(Madore, 2017)
- A countable ordinal is called -stable iff
.
- A countable ordinal is called -stable iff
where is the
least
admissible
ordinal larger than .
- A countable ordinal is called -stable iff
where is the
least admissible ordinal larger than an admissible ordinal larger
than .
- A countable ordinal is called inaccessibly-stable iff
where is the
least computably
inaccessible
ordinal larger than .
- A countable ordinal is called Mahlo-stable iff
where is the
least computably
Mahlo
ordinal larger than ; that is, the least such that
any -recursive function has an
admissible which is closed under .
- A countable ordinal is called doubly -stable iff
there is a -stable ordinal such that
.
- A countable ordinal is called nonprojectible iff the
set of all such that
is unbounded in .
Properties
Any -stable ordinal is stable. This is because
and .
(Jech, 2003) Any -countable stable ordinal is
-stable for the same reason. Therefore, an ordinal is -stable iff
it is -countable and stable. This property is the same for all
variants of stability.
The smallest stable ordinal is also the smallest ordinal such
that ,
which in turn is the smallest ordinal which is not the order-type of any
-ordering of the natural numbers. The smallest stable
ordinal has the property that any
subset of is -finite.
(Madore, 2017)
If there is an ordinal such that
(i.e. the minimal height of a
transitive model of )
then it is smaller than the least stable ordinal. On the other hand, the
sizes of the least -stable ordinal and the least nonprojectible
ordinal lie between the least recursively weakly compact and the least
-admissible (the same for other weakened variants of stability
defined above). (Madore, 2017)
References
- Madore, D. (2017). A zoo of ordinals. http://www.madore.org/ david/math/ordinal-zoo.pdf
- 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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