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Dimension subgroups, nilpotency indices, and the number of generators of ideals in p-group algebras. (English) Zbl 0695.16007
Let K be a field of non-zero characteristic p. Let G be a finite p-group of order $$p^ n$$. Then the augmentation ideal $$\Delta$$ of the group algebra KG is nilpotent, and its index of nilpotence is denoted by t(G). It is well known that $$t(G)\leq p^ n$$, with equality if and only if G is cyclic. S. Koshitani [Tsukuba J. Math. 1, 137-148 (1977; Zbl 0391.16007)] has proved that if G is not cyclic then $$t(G)\leq p^{n-1}- p+1$$, with equality if and only if G contains a cyclic subgroup of index p. Now suppose that $$p\neq 2$$. It is proved that for any $$i>0$$ and sufficiently large n, $$t(G)\geq p^{n-i}$$ if and only if G contains a cyclic subgroup of index $$p^ i$$. Moreover, if $$\exp (G)=p^ e$$, then $$t(G)\leq p^ e+p^{n-e}-1$$, with equality if and only if G is metacyclic.
The Loewy series $$\{c_ i\}$$ is defined by $$c_ i=\dim (\Delta^ i/\Delta^{i+1})$$. It is known that $$\{c_ i\}$$ is not unimodal in general. It is proved that the Loewy series is unimodal if $$n\leq 2e$$, and this generalizes a result of O. Manz and R. Staszewski for metacyclic groups [see their papers in Proc. Am. Math. Soc. 98, 189-195 (1986; Zbl 0611.20003) and J. Reine Angew. Math. 368, 108-118 (1986; Zbl 0586.20006)].
Let v(KG) denote the least upper bound of the minimal numbers of generators of left ideals in KG. It is not difficult to establish that max $$c_ i\leq v(KG)\leq p^{n-e}$$. It is proved that $$v(KG)=p^{n-e}$$ provided 2e$$\geq n$$. Moreover, if G is sufficiently large, then $$v(KG)\leq p^ i$$ if and only if G contains a cyclic subgroup of index $$p^ i.$$
Infinite groups are discussed briefly. Let G be a residually-p group. Then v(KG) is finite if and only if G has a cyclic subgroup of finite index, and in this case $$v(KG)=\min \{[G:C]:$$ C is a cyclic subgroup of $$G\}$$ and v(KG) is a power of p. All of these results are consequences of a detailed study of the dimension subgroups of KG.
Reviewer: P.F.Smith

##### MSC:
 16S34 Group rings 20C05 Group rings of finite groups and their modules (group-theoretic aspects) 20D15 Finite nilpotent groups, $$p$$-groups 16N40 Nil and nilpotent radicals, sets, ideals, associative rings 16Dxx Modules, bimodules and ideals in associative algebras
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