Chang's conjecture

In model theory, a branch of mathematical logic, Chang's conjecture, attributed to Chen Chung Chang by Vaught (1963, p. 309), states that every model of type (ω2,ω1) for a countable language has an elementary submodel of type (ω1, ω). A model is of type (α,β) if it is of cardinality α and a unary relation is represented by a subset of cardinality β. The usual notation is ( ω 2 , ω 1 ) ↠ ( ω 1 , ω ) {\displaystyle (\omega _{2},\omega _{1})\twoheadrightarrow (\omega _{1},\omega )} .

The axiom of constructibility implies that Chang's conjecture fails. Silver proved the consistency of Chang's conjecture from the consistency of an ω1-Erdős cardinal. Hans-Dieter Donder showed a weak version of the reverse implication: if CC is not only consistent but actually holds, then ω2 is ω1-Erdős in K.

More generally, Chang's conjecture for two pairs (α,β), (γ,δ) of cardinals is the claim that every model of type (α,β) for a countable language has an elementary submodel of type (γ,δ). The consistency of ( ω 3 , ω 2 ) ↠ ( ω 2 , ω 1 ) {\displaystyle (\omega _{3},\omega _{2})\twoheadrightarrow (\omega _{2},\omega _{1})} was shown by Laver from the consistency of a huge cardinal.

References

  • Chang, Chen Chung; Keisler, H. Jerome (1990), Model Theory, Studies in Logic and the Foundations of Mathematics (3rd ed.), Elsevier, ISBN 978-0-444-88054-3
  • Vaught, R. L. (1963), "Models of complete theories", Bulletin of the American Mathematical Society, 69: 299–313, doi:10.1090/S0002-9904-1963-10903-9, ISSN 0002-9904, MR 0147396


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