A theory of pairs for non-valuational structures

Abstract

Given a weakly o-minimal structure $\mathcal M$ and its o-minimal completion $\bar {\mathcal M}$, we first associate to $\bar {\mathcal M}$ a canonical language and then prove that $Th(\mathcal M)$ determines $Th(\bar {\mathcal M})$. We then investigate the theory of the pair $\mathcal M^P=(\bar {\mathcal M};M)$ in the spirit of the theory of dense pairs of o-minimal structures, and prove, among other results, that it is near model complete, and every $\mathcal M^P$-definable open subset of $\bar M^n$ is already definable in $\bar {\mathcal M}$. We give an example of a weakly o-minimal structure which interprets $\bar {\mathcal M}$ and show that it is not elementarily equivalent to any reduct of an o-minimal trace.