# Subnormal series

(Redirected from Attainable subgroup)

2010 Mathematics Subject Classification: Primary: 20D30 Secondary: 20D35 [MSN][ZBL]

A subnormal series (or subinvariant series) of a group \$G\$ is a subgroup series \$\$ E = G_0 \le G_1 \le \cdots \le G_n = G \$\$ in which each subgroup \$G_i\$ is a normal subgroup of \$G_{i+1}\$. The quotient groups \$G_{i+1}/G_i\$ are called factors, and the number \$n\$ is called the length of the subnormal series. Infinite subnormal series have also been studied (see Subgroup system).

A subnormal series that cannot be refined further is called a composition series, and its factors are called composition factors. Any two subnormal series of a group have isomorphic refinements and in particular, any two composition series are isomorphic (see Jordan–Hölder theorem).

A subnormal subgroup (also subinvariant, attainable or accessible) of \$G\$ is a subgroup that appears in some subnormal series of \$G\$. To indicate the subnormality of a subgroup \$H\$ in a group \$G\$, the notation \$H \lhd\!\lhd G\$ is used.

The property of a subgroup to be subnormal is transitive. An intersection of subnormal subgroups is again a subnormal subgroup. The subgroup generated by two subnormal subgroups need not be subnormal. A group \$G\$ all subgroups of which are subnormal satisfies the normalizer condition, i.e. all subgroups differ from their normalizers (cf. Normalizer of a subset). Such a group is therefore locally nilpotent.

A subnormal subgroup of \$G\$ that coincides with its commutator subgroup and whose quotient by its centre is simple is called a component of \$G\$. The product of all components of \$G\$ is known as the layer of \$G\$. It is an important characteristic subgroup of \$G\$ in the theory of finite simple groups, see e.g. [6].

#### References

 [1] M.I. Kargapolov, J.I. [Yu.I. Merzlyakov] Merzljakov, "Fundamentals of the theory of groups" , Springer (1979) (Translated from Russian) [2] A.G. Kurosh, "The theory of groups" , 1–2 , Chelsea (1955–1956) (Translated from Russian) [3] M. Hall jr., "The theory of groups" , Macmillan (1959) pp. Sect. 8.4 [4] J.C. Lennox, S.E. Stonehewer, "Subnormal subgroups of groups" , Clarendon Press (1987) [5] D.J.S. Robinson, "A course in the theory of groups" , Springer (1982) [6] M. Suzuki, "Group theory" , 1–2 , Springer (1986)
How to Cite This Entry:
Attainable subgroup. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Attainable_subgroup&oldid=51197
This article was adapted from an original article by V.M. Kopytov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article