Imaginaries, products and the adele ring

Abstract

We describe the imaginary sorts of infinite products in terms of imaginary sorts of the factors (Theorem 3.1). We extend the result to certain reduced powers (Theorem 4.1) and then to infinite products ${\prod }_{i\in I}{M}_i$ enriched with a predicate for the ideal of finite subsets of $I$ (Theorem 5.1). As a special case, using the Hils–Rideau-Kikuchi uniform $p$-adic elimination of imaginaries, we find the imaginary sorts of the ring of rational adeles (Example 5.1). Our methods include the use of the Harrington–Kechris–Louveau Glimm–Effros dichotomy both for transitioning from monadic second order imaginaries to first-order reducts, and for proving a certain “one-way” model-theoretic orthogonality within the adelic imaginaries.