
Given an Abelian group \(G\), the group radical is defined by \(R_G(X)=\bigcap\{\text{Ker }\phi\mid\phi\colon X\to G\}\), for Abelian groups \(X\). This radical does not always commute with infinite direct products (for instance, when \(G=\mathbb{Q}\), it turns into the torsion radical). Given any radical \(R\) in the category of Abelian groups, its norm \(\|R\|\) is defined to be the least cardinal \(\kappa\) for which \(R\) does not always commute with \(\kappa\)-direct products of groups. If this cardinal exists it has to be regular. Assuming GCH, if \(\kappa\) is any uncountable regular cardinal with the property that no cardinal \(<\kappa\) is weakly compact, then there exists a torsion-free group \(G\) of cardinality \(2^\kappa\), with \(\|R\|=\kappa\). The coslender radical is a non-trivial example of a radical with norm equal to the first non-measurable cardinal [see \textit{R. Dimitrić}, Proc. Conf., Hobart/Aust. 1987, Pitman Res. Notes Math. Ser. 204, 41-50 (1989; Zbl 0671.20053)].
group radicals, Automorphisms, homomorphisms, endomorphisms, etc. for abelian groups, Continuum hypothesis and Martin's axiom, Subgroups of abelian groups, radical norms, rigid systems, Direct sums, direct products, etc. for abelian groups, categories of Abelian groups, radicals commuting with infinite products, regular cardinals, reduced products
group radicals, Automorphisms, homomorphisms, endomorphisms, etc. for abelian groups, Continuum hypothesis and Martin's axiom, Subgroups of abelian groups, radical norms, rigid systems, Direct sums, direct products, etc. for abelian groups, categories of Abelian groups, radicals commuting with infinite products, regular cardinals, reduced products
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