Trivial Isomorphisms between Reduced Products

Abstract

We introduce a general method for showing under weak forcing axioms that reduced products of countable models of a theory $T$ have as few automorphisms as possible. We show that such forcing axioms imply that reduced products of countably infinite or finite fields, linear orders, trees, or random graphs have only trivial automorphisms. We also show that Todorčević's Open Colouring Axiom, ${\mathsf{OCA}}_{\mathrm{T}}$, implies that all automorphisms of $\mathcal{P}(\mathbb{N})/\mathrm{Fin}$ are trivial.

44 pages. Correction and simplifications in Section 5 (in particular Proposition 5.6), other minor corrections and improvements