The generic smooth expansion conjecture for semialgebraic Mordell–Lang circle groups

Let II be a closed bounded interval with nonempty interior, and let C(I)C^\infty(I) be the topological vector space of smooth functions IRI\to\mathbb{R}. Let H\mathbb{H} be a semialgebraic Mordell–Lang circle group, let γ:R/ZH\gamma:\mathbb{R}/\mathbb{Z}\to\mathbb{H} be the unique up to sign topological group isomorphism, let αR/Z\alpha\in\mathbb{R}/\mathbb{Z} be irrational, and define

χ(k)=γ(αk),A:=χ(Z).\chi(k)=\gamma(\alpha k),\qquad A:=\chi(\mathbb{Z}).

Generic smooth expansion conjecture. There is a comeager subset ΛC(I)\Lambda\subseteq C^\infty(I), possibly depending on α\alpha, such that for every fΛf\in\Lambda:

  1. (R,+,×,f)(\mathbb{R},+,\times,f) is o-minimal;
  2. if fgf\ne g are in Λ\Lambda, then (R,+,×,f)(\mathbb{R},+,\times,f) and (R,+,×,g)(\mathbb{R},+,\times,g) are not interdefinable;
  3. every (R,+,×,f)(\mathbb{R},+,\times,f)-definable group is definably isomorphic to a semialgebraic group, and every definable homomorphism between semialgebraic groups is semialgebraic;
  4. (R,+,×,A)(\mathbb{R},+,\times,A) is NIP and Th(R,+,×)\operatorname{Th}(\mathbb{R},+,\times) is an open core of Th(R,+,×,A)\operatorname{Th}(\mathbb{R},+,\times,A);
  5. every (R,+,×,A)(\mathbb{R},+,\times,A)-definable subset of AkA^k is a finite union of sets bn(XAk)b\oplus n(X\cap A^k), with XX semialgebraic and bAkb\in A^k.

In particular, the structure induced on AA by (R,+,×)(\mathbb{R},+,\times) is a dp-minimal expansion of (Z,+,Cα)(\mathbb{Z},+,C_\alpha).

Sources & referencesView supporting material

Primary source

Erik Walsberg, “Dp-minimal expansions of (Z,+) via dense pairs via Mordell-Lang”, arXiv:2004.06847 (2020).

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