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When are profinite many-sorted algebras retracts of ultraproducts of finite many-sorted algebras?

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	Climent Vidal, J.; Cosme i Llópez, Enric
	This document is a artículoDate2018
	Este documento está disponible también en : http://hdl.handle.net/10550/69537
	For a set of sorts S and an S-sorted signature Σ we prove that a profinite Σ-algebra, i.e. a projective limit of a projective system of finite Σ-algebras, is a retract of an ultraproduct of finite Σ-algebras if the family consisting of the finite Σ-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.
	Climent Vidal, J. Cosme i Llópez, Enric 2018 When are profinite many-sorted algebras retracts of ultraproducts of finite many-sorted algebras? Logic Journal of the IGPL 26 4 381 407
	https://doi.org/10.1093/jigpal/jzy005