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Acta Mathematica Hungarica
Article . 1984 . Peer-reviewed
License: Springer TDM
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image/svg+xml Jakob Voss, based on art designer at PLoS, modified by Wikipedia users Nina and Beao Closed Access logo, derived from PLoS Open Access logo. This version with transparent background. http://commons.wikimedia.org/wiki/File:Closed_Access_logo_transparent.svg Jakob Voss, based on art designer at PLoS, modified by Wikipedia users Nina and Beao
zbMATH Open
Article . 1984
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Projective classes of torsion free abelian groups. II

Projective classes of torsion free Abelian groups. II
Authors: Wickless, W. J.;

Projective classes of torsion free abelian groups. II

Abstract

[Part I, cf. the author and \textit{C. Vinsonhaler}, ibid. 39, 195-215 (1982; Zbl 0496.20041).] Let \({\mathcal C}\) be the category of torsion free abelian groups of finite rank, \(G\in {\mathcal C}\), \(C_ 0(G)=\{A\in {\mathcal C}|\) G is projective with respect to all \({\mathcal C}\) exact sequences (pure exact sequences) of the form: \(0\to K\to A\to A/K\to 0\}, P_ 0C_ 0(G)=\{Y\in {\mathcal C}|\) Y is projective with respect to all \({\mathcal C}\) exact sequences: \(0\to K\to A\to A/K\to 0, \forall A\in C_ 0(G)\}\). For \(X=\{X_{\alpha}\}\), a set of groups, \(C_ 0(X)=\cap_{\alpha}C_ 0(X_{\alpha})\) and \(P_ 0C_ 0(X)=\{Y\in {\mathcal C}|\) Y is projective with respect to all \(0\to K\to A\to A/K\to 0, \forall A\in {\mathcal C}_ 0(X)\}\). Let \(G\in {\mathcal C}\) and let \({\mathcal S}_ G\) be the set of all rank one factors of G. The cotypeset of G is the set of all types of torsion free rank one factors of G. Theorem 1. Let G be a torsion free abelian group of finite rank whose cotypeset contains only idempotent types. Then \(P_ 0C_ 0(G)=P_ 0C_ 0({\mathcal S}_ G)\). - If \(X=\{X_{\alpha}\}\) is a set of groups in \({\mathcal C}\) let \({\mathcal I}(X)=\{W\in {\mathcal C}|\quad Ext(X_{\alpha},W)=(0),\quad\forall X_{\alpha}\in X\}\) and let \({\mathcal P}{\mathcal I}(X)=\{Y\in {\mathcal C}| Ext(Y,W)=(0),\quad\forall W\in {\mathcal I}(X)\}. {\mathcal P}{\mathcal I}(X)\) is called the cotorsion-free class of X in \({\mathcal C}\). (It satisfies the requirements for an abstract cotorsion free class.) Let \(\Pi\) be the set of all primes, S an arbitrary non-empty subset of \(\Pi\) and \(\tau\) an arbitrary type. Write \(\tau_{\tilde S}\bar O\) if there exists \(h\in\tau \) such that \(h(p)=0\) for all \(p\in S\). Here \(\bar O=type Z\). If \(\{\tau_{\alpha}\}\) is a set of types let \(\{\tau_{\alpha}\}^ 0=\{S\subseteq\Pi | \tau_{\alpha\tilde S}\bar O\), \(\forall\alpha \}\), \(\{ {\bar\tau }{}_{\alpha}\}=\{types \tau| \tau_{\tilde S}\bar O\), \(\forall S\in\{\tau_{\alpha}\}^ 0\}.\) Theorem 2. Let \(X=\{X_{\alpha}\}\) be a set of groups in \({\mathcal C}\). Then, for \(Y\in {\mathcal C}\), \(Y\in {\mathcal P}{\mathcal I}(X)\) iff \(OT(Y)\in\{OT(X_{\alpha})\}\). - Corollary. Let \(G\in {\mathcal C}\). Then \({\mathcal P}{\mathcal I}(G)={\mathcal P}{\mathcal I}({\mathcal S}_ G)\). - Theorem 3. Let \(G\in {\mathcal C}\). Then \(P_ 0C_ 0\subseteq {\mathcal P}{\mathcal I}(G)\). - Let cotypeset \({}^*G=\{t\in\cot ypeset G| t>\bar O\}\), \([\tau]=\{p\in\Pi | h(p)=\infty\}\). Theorem 4. Let \(G\in {\mathcal C}\). Then \(P_ 0C_ 0(G)={\mathcal P}{\mathcal I}(G)\) iff there exists an idempotent type \(\tau\) with [\(\tau]\) finite such that cotypeset \(^*G=\{\)types \(\tau'|\) \(\bar O<\tau'\leq\tau\}\). - Remark. In this case \(\tau =OT(G)\) and cotypeset G is necessarily finite.

Related Organizations
Keywords

Torsion-free groups, finite rank, Homological and categorical methods for abelian groups, idempotent types, cotypeset, cotorsion-free class, category of torsion free abelian groups of finite rank, pure exact sequences

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
11
Average
Top 10%
Average
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