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Groups with restriction on their infinite subnormal subgroups. (English) Zbl 0644.20020
A group G is called a $$B_ n$$-group if all its subnormal subgroups have defect at most n. If all infinite subnormal subgroups of G have defect at most n, G is called an $$IB_ n$$-group. The structure of soluble $$IB_ n$$-groups was studied by F. de Giovanni and the reviewer [J. Algebra 96, 566-580 (1985; Zbl 0572.20016); Boll. Unione Mat. Ital., VI. Ser., D, Algebra Geom. 4, No.1, 49-56 (1985; Zbl 0605.20026)]. Here the author considers $$IB_ n$$-groups in general, especially in comparison to $$B_ n$$-groups. The following is proved: If G is an infinite $$IB_ n$$- group, there exists an abelian normal subgroup K of G such that K is either finite or a Prüfer group and G/K is a $$B_ n$$-group. Also, every $$IB_ n$$-group is a $$B_{n+1}$$-group.
B$${}_ 1$$-groups are those in which normality is a transitive relation (T-groups), and $$IB_ 1$$-groups are those in which every infinite subnormal subgroup is normal (IT-groups). For IT-groups the following is proved: If G is an infinite IT-group, it is an extension of a T-group by a residually finite T-group, which is either metabelian or abelian-by- finite of finite exponent. Examples are given to show that some statements on IT-groups cannot be strengthened. Finally, it is pointed out that the groups constructed in one of these examples are counterexamples to a theorem by W. Gaschütz [J. Reine Angew. Math. 198, 87-92 (1957; Zbl 0077.250)].
Reviewer: S.Franciosi

##### MSC:
 2e+16 Chains and lattices of subgroups, subnormal subgroups 2e+08 Subgroup theorems; subgroup growth 2e+35 General structure theorems for groups
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##### References:
 [1] DOI: 10.1007/BF01580283 · Zbl 0021.21003 [2] DOI: 10.1017/S0305004100037403 [3] DOI: 10.1016/0021-8693(85)90027-4 · Zbl 0572.20016 [4] Gaschütz, J. Reine. Angew. Math. 198 pp 87– (1957) [5] De Giovanni, Boll. Un. Mat. Ital. D 4 pp 49– (1985) [6] Kassens, Die Wielandtlänge endlicher Gruppen (1980)
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