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On countable extensions of primary abelian groups
Archivum mathematicum, Tome 43 (2007) no. 1, pp. 61-66 Cet article a éte moissonné depuis la source Czech Digital Mathematics Library

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It is proved that if $A$ is an abelian $p$-group with a pure subgroup $G$ so that $A/G$ is at most countable and $G$ is either $p^{\omega +n}$-totally projective or $p^{\omega +n}$-summable, then $A$ is either $p^{\omega +n}$-totally projective or $p^{\omega +n}$-summable as well. Moreover, if in addition $G$ is nice in $A$, then $G$ being either strongly $p^{\omega +n}$-totally projective or strongly $p^{\omega +n}$-summable implies that so is $A$. This generalizes a classical result of Wallace (J. Algebra, 1971) for totally projective $p$-groups as well as continues our recent investigations in (Arch. Math. (Brno), 2005 and 2006). Some other related results are also established.
It is proved that if $A$ is an abelian $p$-group with a pure subgroup $G$ so that $A/G$ is at most countable and $G$ is either $p^{\omega +n}$-totally projective or $p^{\omega +n}$-summable, then $A$ is either $p^{\omega +n}$-totally projective or $p^{\omega +n}$-summable as well. Moreover, if in addition $G$ is nice in $A$, then $G$ being either strongly $p^{\omega +n}$-totally projective or strongly $p^{\omega +n}$-summable implies that so is $A$. This generalizes a classical result of Wallace (J. Algebra, 1971) for totally projective $p$-groups as well as continues our recent investigations in (Arch. Math. (Brno), 2005 and 2006). Some other related results are also established.
Classification : 20K10, 20K15
Keywords: countable quotient groups; $\omega $-elongations; $p^{\omega +n}$-totally projective groups; $p^{\omega +n}$-summable groups 
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Danchev, P. V. On countable extensions of primary abelian groups. Archivum mathematicum, Tome 43 (2007) no. 1, pp. 61-66. http://geodesic.mathdoc.fr/item/ARM_2007_43_1_a5/

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