On the ‘definability of definable’ problem of Alfred Tarski, Part II

Author:

Kanovei Vladimir,Lyubetsky Vassily

Abstract

Alfred Tarski [J. Symbolic Logic 13 (1948), pp. 107–111] defined D p m \mathbf {D}_{pm} to be the set of all sets of type p p , type-theoretically definable by parameterfree formulas of type m {\le m} , and asked whether it is true that D 1 m D 2 m \mathbf {D}_{1m}\in \mathbf {D}_{2m} for m 1 m\ge 1 . Tarski noted that the negative solution is consistent because the axiom of constructibility V = L \mathbf {V}=\mathbf {L} implies D 1 m D 2 m \mathbf {D}_{1m}\notin \mathbf {D}_{2m} for all m 1 m\ge 1 , and he left the consistency of the positive solution as a major open problem. This was solved in our recent paper [Mathematics 8 (2020), pp. 1–36], where it is established that for any m 1 m\ge 1 there is a generic extension of L \mathbf {L} , the constructible universe, in which it is true that D 1 m D 2 m \mathbf {D}_{1m}\in \mathbf {D}_{2m} . In continuation of this research, we prove here that Tarski’s sentences D 1 m D 2 m \mathbf {D}_{1m}\in \mathbf {D}_{2m} are not only consistent, but also independent of each other, in the sense that for any set Y ω { 0 } Y\subseteq \omega \smallsetminus \{0\} in L \mathbf {L} there is a generic extension of L \mathbf {L} in which it is true that D 1 m D 2 m \mathbf {D}_{1m}\in \mathbf {D}_{2m} holds for all m Y m\in Y but fails for all m 1 m\ge 1 , m Y m\notin Y . This gives a full and conclusive solution of the Tarski problem.

The other main result of this paper is the consistency of D 1 D 2 \mathbf {D}_{1}\in \mathbf {D}_{2} via another generic extension of L \mathbf {L} , where D p = m D p m \mathbf {D}_{p}=\bigcup _m\mathbf {D}_{pm} , the set of all sets of type p p , type-theoretically definable by formulas of any type.

Our methods are based on almost-disjoint forcing of Jensen and Solovay [Some applications of almost disjoint sets, North-Holland, Amsterdam, 1970, pp. 84–104].

Funder

Russian Foundation for Basic Research

Publisher

American Mathematical Society (AMS)

Subject

Applied Mathematics,General Mathematics

Reference52 articles.

1. J. W. Addison, The undefinability of the definable, Not. AMS 12 (1965), no. 3, 347–348.

2. Tarski’s theory of definability: common themes in descriptive set theory, recursive function theory, classical pure logic, and finite-universe logic;Addison, J. W.;Ann. Pure Appl. Logic,2004

3. Hyperclass forcing in Morse-Kelley class theory;Antos, Carolin;J. Symb. Log.,2017

4. The Hyperuniverse Project and Maximality

5. J. Barwise (ed.), Handbook of mathematical logic, Studies in Logic and the Foundations of Mathematics, vol. 90, North-Holland, Amst., 1977. \MR{MR709106 (84g:03004a)}

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