When are profinite many-sorted algebras retracts of ultraproducts of finite many-sorted algebras?
J Climent Vidal, Enric Cosme Llópez · Logic Journal of IGPL · 2018
For a set of sorts S and an S-sorted signature |$\Sigma $| we prove that a profinite |$\Sigma $|-algebra, i.e. a projective limit of a projective system of finite |$\Sigma $|-algebras, is a retract of an ultraproduct of finite |$\Sigma $|-algebras if the family consisting of the finite |$\Sigma $|-algebras underlying the projective system is with constant support. In addition, we provide a categorial rendering of the above result. Specifically, after obtaining a category where the objects are the pairs formed by a nonempty upward directed preordered set and by an ultrafilter containing the filter of the final sections of it, we show that there exists a functor from the just mentioned category whose object mapping assigns to an object a natural transformation which is a retraction.