Hodges's conjecture on relative categoricity and the Gaifman property

Let TT be a complete theory with quantifier elimination in a relational language LL, and let PP be a distinguished unary predicate symbol in LL. For a model MTM\models T, write MPM^{P} for the LL-substructure of MM with universe P(M)P(M), and let TPT^{P} be the common complete theory of these structures as MM ranges over models of TT. The theory TT is relatively categorical if every isomorphism between the PP-parts of two models of TT lifts to an isomorphism between the models. Hodges's conjecture. If TT is relatively categorical, then TT has the Gaifman property. The source indicates that this conjecture is refuted by a counterexample, so the claim is not valid in general.

Sources & referencesView supporting material

Primary source

Anand Pillay, “Remarks on relative categoricity”, arXiv:2602.05866 (2026).

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