Finite-vocabulary obstruction to elimination of imaginaries in Hrushovski constructions

Let τ\tau be a finite relational vocabulary, and let K0\boldsymbol{K}_0 be the class of all finite τ\tau-structures satisfying the hereditarily positive ϵ\epsilon dimension from Axiom 2. Let δ\delta be Hrushovski's predimension, let (A/B)(A/B) be a good pair, and let sdcl(I)\operatorname{sdcl}^*(I) denote the symmetric definable closure modulo definable closure of proper subsets. Assume that there is a natural number NN such that

μ(A/B)δ(B)\mu(A/B)\geqslant\delta(B)

for every good pair (A/B)(A/B) with δ(B)N\delta(B)\geqslant N.

Finite-vocabulary conjecture. Under these assumptions,

sdcl(I)=\operatorname{sdcl}^*(I)=\emptyset

for every independent set II with Imax{N,5}|I|\geqslant\max\{N,5\}. Consequently, no Hrushovski construction in a finite relational vocabulary τ\tau—that is, with K0\boldsymbol{K}_0 containing all finite τ\tau-structures—has elimination of imaginaries.

This is the source's exact formulation of the proposed sufficient condition. It would explain why the infinite-vocabulary construction can have elimination of imaginaries while finite relational vocabularies cannot; the source provides no resolution.

Sources & referencesView supporting material

Primary source

John T. Baldwin and Viktor V. Verbovskiy, “Towards a Finer Classification of Strongly Minimal Sets”, arXiv:2106.15567 (2024).

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