Abstract
Let R be a sufficiently saturated o-minimal expansion of a real closed field, let \( \mathcal{O} \) be the convex hull of ℚ in R, and let st: \( \mathcal{O}^n \) → ℝn be the standard part map. For X ⊆ R n define st X:= st (X ∩ \( \mathcal{O}^n \)). We let ℝind be the structure with underlying set ℝ and expanded by all sets st X, where X ⊆ R n is definable in R and n = 1, 2,.... We show that the subsets of ℝn that are definable in ℝind are exactly the finite unions of sets st X st Y, where X, Y ⊆ R n are definable in R. A consequence of the proof is a partial answer to a question by Hrushovski, Peterzil and Pillay about the existence of measures with certain invariance properties on the lattice of bounded definable sets in R n.
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Maříková, J. The structure on the real field generated by the standard part map on an o-minimal expansion of a real closed field. Isr. J. Math. 171, 175–195 (2009). https://doi.org/10.1007/s11856-009-0046-5
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DOI: https://doi.org/10.1007/s11856-009-0046-5