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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 (ω21) 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 (\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 the reverse implication: if CC holds, then ω2 is ω1-Erdõs 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 (\omega_3,\omega_2)\twoheadrightarrow(\omega_2,\omega_1) was shown by Laver from the consistency of a huge cardinal.

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