In this paper we study elimination of imaginaries in some classes of pure ordered abelian groups. For the class of ordered abelian groups with bounded regular rank (equivalently with finite spines) we obtain weak elimination of imaginaries once we add sorts for the quotient groups Γ ∆ for each definable convex subgroup ∆, and sorts for the quotient groups Γ ∆ + lΓ where ∆ is a definable convex subgroup and l ∈ N≥2. We refer to these sorts as the quotient sorts. For the dp-minimal case we obtain a complete elimination of imaginaries, if we also add constants to distinguish the cosets of ∆ + nΓ in Γ, where ∆ is a definable convex subgroup and n ∈ N≥2.Aknowledgements: This document was written as part of the author's PhD thesis under Pierre Simon and Thomas Scanlon. The author would like to express her gratitude to both of them, for many insightful conversations and their time. The author would like to thank particularly Pierre for his continuous encouragement and his financial support from his NSF Grant 1848562. The author would like to thank as well R. Mennuni for some useful comments in a previous draft.
Preliminaries
Elimination of imaginariesLet T be a first order theory and M be its monster model. Let D ⊆ M k be some definable set and E some definable equivalence relation over D. The equivalence class e = a E is said to be an imaginary element. Imaginaries in model theory were introduced by Shelah in [14]. Later in [15], Makkai proposed to construct the many sorted structure M eq , where we add a sort S E for each definable equivalence relation E and a map π E sending each element to its class. Since then, the model theoretic community has presented and studied imaginary elements in this way and refer to the multi-sorted structure M eq , as the imaginary expansion of M. We call the sorts S E as imaginary sorts while we refer to M as the home-sort. Any formula φ(x, ȳ) induces an equivalence relation in M ȳ defined asLet b ∈ M ȳ and X ∶= φ(x, b). We call the class b E φ the code of X and denote it as ⌜X⌝. We denote as dcl eq and acl eq the definable closure and the algebraic closure in the expansion M eq . Definition 2.1.1. We say that T has elimination of imaginaries if for any imaginary element e there is a tuple a in the home-sort such that e ∈ dcl eq (a) and a ∈ dcl eq (e).2. We say that T has weak elimination of imaginaries if for any imaginary element e there is a tuple a in the home-sort such that e ∈ dcl eq (a) and a ∈ acl eq (e).3. We say that T codes finite sets if for every model M ⊧ T , and every set S of a finitely many elements in M , the code ⌜S⌝ is interdefinable with a tuple of elements in M .The following is a folklore fact.Fact 2.2. Let T be a complete multi-sorted theory. If T has weak elimination of imaginaries and codes finite sets then T eliminates imaginaries.