Structure theorems in tame expansions of o-minimal structures by a dense set

Abstract

We study sets and groups definable in tame expansions of o-minimal structures. Let $\tilde{\mathcal{M}}=⟨\mathcal{M},P⟩$ be an expansion of an o-minimal $\mathcal{L}$-structure $\mathcal{M}$ by a dense set $P$, such that three tameness conditions hold. We prove a structure theorem for definable sets and functions in analogy with the influential cell decomposition theorem known for o-minimal structures. The structure theorem advances the state-of-the-art in all known examples of $\tilde{\mathcal{M}}$, as it achieves a decomposition of definable sets into \emph{unions} of `cones', instead of only boolean combinations of them. We also develop the right dimension theory in the tame setting. Applications include: (i) the dimension of a definable set coincides with a suitable pregeometric dimension, and it is invariant under definable bijections, (ii) every definable map is given by an $\mathcal{L}$-definable map off a subset of its domain of smaller dimension, and (iii) around generic elements of a definable group, the group operation is given by an $\mathcal{L}$-definable map.