
A subgroup \(H\) of an abelian group \(G\) is called fully inert if the quotient \((H+\phi(H))/H\) is finite for every endomorphism \(\phi\) of \(G\). This is a common generalization of the notions of fully invariant, finite and finite-index subgroups. Two subgroups \(K\) and \(L\) of \(G\) are commensurable if \([K:L\cap K]\) and \([L:L\cap K]\) are both finite. \(\phi\)-inert subgroups were a basic tool in the definition of intrinsic algebraic entropy introduced in [\textit{D. Dikranjan} et al., J. Pure Appl. Algebra 219, No. 7, 2933--2961 (2015; Zbl 1155.20041)], which in turn is a variant of the notion of algebraic entropy that was investigated in depth in [\textit{D. Dikranjan} et al., Trans. Am. Math. Soc. 361, No. 7, 3401--3434 (2009; Zbl 1176.20057)]. The authors states that fully inert subgroups of torsion-complete abelian \(p\)-groups are commensurable with fully invariant subgroups. This result is analogues to corresponding statement for direct sums of \(p\)-cyclic groups [\textit{B. Goldsmith} et al., J. Algebra 419, 332--349 (2014; Zbl 1305.20063)].
fully inert subgroup, Algebra and Number Theory, commensurable subgroups, Direct sums, direct products, etc. for abelian groups, Automorphisms, homomorphisms, endomorphisms, etc. for abelian groups, direct sum of cyclic \(p\)-groups, Subgroups of abelian groups, fully invariant subgroup, Torsion groups, primary groups and generalized primary groups, torsion-complete \(p\)-group
fully inert subgroup, Algebra and Number Theory, commensurable subgroups, Direct sums, direct products, etc. for abelian groups, Automorphisms, homomorphisms, endomorphisms, etc. for abelian groups, direct sum of cyclic \(p\)-groups, Subgroups of abelian groups, fully invariant subgroup, Torsion groups, primary groups and generalized primary groups, torsion-complete \(p\)-group
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