Problem #18

Scott sets and standard systems of models of PA

Open!Ordinary impact — every problem listed here is a genuine, worthwhile open problem

Is every Scott set the standard system of a non-standard model of PA\mathsf{PA}?

Definitions

  • A Turing ideal is a set A2NA \subseteq 2^\mathbb{N} which is closed under finite joins and Turing reducibility. I.e. for any x,yAx, y \in A, xyx\oplus y is in AA as is any zTxz \leq_T x.
  • A Scott set is a Turing ideal AA such that for any xAx \in A which codes an infinite binary tree, AA contains some element which codes an infinite path through this tree.
  • Given a non-standard model MM of PA\mathsf{PA}, the standard system of MM, denoted SSy(M)\mathsf{SSy}(M), is the set of reals x2Nx \in 2^\mathbb{N} such that xx is coded by some element of MM, i.e. there is some non-standard aMa \in M such that x(n)=1x(n) = 1 if and only if the nthn^\text{th} prime number divides aa.

Known Partial Results

  • Scott [Sco62] proved that every countable Scott set is the standard system of a non-standard model of PA\mathsf{PA}.
  • Knight and Nadel [KN82] proved that every size 1\aleph_1 Scott set is the standard system of a non-standard model of PA\mathsf{PA}. Hence the question has a positive answer under CH\mathsf{CH}.
  • Gitman [Git08] proved that under the Proper Forcing Axiom (PFA\mathsf{PFA}), Scott sets of any size which satisfy certain technical conditions (arithmetically closed and proper) are the standard system of a non-standard model of PA\mathsf{PA}.

Notes

Scott [Sco62] proved that every if MM is a non-standard model of PA\mathsf{PA} 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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