Problem #18
Scott sets and standard systems of models of PA
Is every Scott set the standard system of a non-standard model of ?
Definitions
- A Turing ideal is a set which is closed under finite joins and Turing reducibility. I.e. for any , is in as is any .
- A Scott set is a Turing ideal such that for any which codes an infinite binary tree, contains some element which codes an infinite path through this tree.
- Given a non-standard model of , the standard system of , denoted , is the set of reals such that is coded by some element of , i.e. there is some non-standard such that if and only if the prime number divides .
Known Partial Results
- Scott [Sco62] proved that every countable Scott set is the standard system of a non-standard model of .
- Knight and Nadel [KN82] proved that every size Scott set is the standard system of a non-standard model of . Hence the question has a positive answer under .
- Gitman [Git08] proved that under the Proper Forcing Axiom (), Scott sets of any size which satisfy certain technical conditions (arithmetically closed and proper) are the standard system of a non-standard model of .
Notes
Scott [Sco62] proved that every if is a non-standard model of then its standard system is a Scott set. This question asks whether the converse holds.
Reference for the problem statement
[Wan26]Wei Wang, Some notes on uncountable models of arithmetic, Fundamenta Mathematicae, 2026 [link] [doi]
Additional References
[Sco62]Dana Scott, Algebras of sets binumerable in complete extensions of arithmetic, Proc. Sympos. Pure Math., Vol. V, 1962
[KN82]Julia Knight and Mark Nadel, Models of arithmetic and closed ideals, Journal of Symoblic Logic, 1982 [doi]
[Git08]Victoria Gitman, Scott's problem for proper Scott sets, Journal of Symbolic Logic, 2008 [doi]
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