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Specker's theorem for Nöbeling's group
Authors:Andreas Blass
Affiliation:Department of Mathematics, University of Michigan, Ann Arbor, Michigan 48109--1109
Abstract:Specker proved that the group $mathbb{Z} ^{aleph_0}$ of integer-valued sequences is far from free; all its homomorphisms to $mathbb{Z} $ factor through finite subproducts. Nöbeling proved that the subgroup $mathcal{B}$ consisting of the bounded sequences is free and therefore has many homomorphisms to $mathbb{Z} $. We prove that all ``reasonable' homomorphisms $mathcal{B}tomathbb{Z} $ factor through finite subproducts. Among the reasonable homomorphisms are all those that are Borel with respect to a natural topology on $mathcal{B}$. In the absence of the axiom of choice, it is consistent that all homomorphisms are reasonable and therefore that Specker's theorem applies to $mathcal{B}$as well as to $mathbb{Z} ^{aleph_0}$.

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