English

Enclosing a Compact Set in an O-minimal Expansion of $(\mathbb{R},+,\cdot,0,1 <)$

Logic 2026-07-27 v1

Abstract

Fix an o-minimal expansion R=(R,+,,0,1<,...)\mathcal{R}=(\mathbb{R},+,\cdot,0,1 <,...) of the real ordered field. Given C1C^1 functions f1,...,fkf_1,...,f_k, g1,...,gk:MRg_1,...,g_k:M\to\mathbb{R} on a definable cell MM, let hi,0h_{i,0} denote fif_i and hi,1h_{i,1} denote gig_i. Suppose that for all τ2[k]\tau\in 2^{[k]}, Hτ=(h1,τ(1),..,hk,τ(k)):MRkH_{\tau}=(h_{1,\tau(1)},..,h_{k,\tau(k)}) :M\to \mathbb{R}^k is regular and proper on MM, and that for all i[k]i\in[k], {fi=0}\{f_i=0\} and {gi=0} \{g_i=0\} are connected, and {fi=0}{gi=0}=\{f_i=0\}\cap \{g_i=0\}=\emptyset. We show that then there exists a sequence (i,ϵ:i[k],ϵ{0,1}){,}[k]×{0,1}(\square_{i,\epsilon}:i\in[k],\epsilon \in \{0,1\})\in\{\leq ,\geq \}^{[k]\times\{0,1\}} such that the enclosed region i[k]{fii,00}{gii,10}\underset{i\in[k]}{\bigcap}\{ f_i\square_{i,0} 0\}\cap \{ g_i\square_{i,1} 0\} is compact.

Keywords

Cite

@article{arxiv.2607.24627,
  title  = {Enclosing a Compact Set in an O-minimal Expansion of $(\mathbb{R},+,\cdot,0,1 <)$},
  author = {Yayi Fu},
  journal= {arXiv preprint arXiv:2607.24627},
  year   = {2026}
}