# Abnormal subgroup

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A subgroup $A$ of a group $G$ such that $g\in\langle A,A^g\rangle$ for any element $g\in G$. Here $\langle A,A^g\rangle$ is the subgroup generated by $A$ and its conjugate subgroup $A^g=gAg^{-1}$. As an example of an abnormal subgroup of a finite group $G$ one can take the normalizer (cf. Normalizer of a subset) $N_G(P)$ of any Sylow $p$-subgroup $P\subset G$, and even any maximal subgroup $N\subset G$ which is not normal in $G$. In the theory of finite solvable groups (cf. Solvable group), where many important classes of subgroups are abnormal, use is made of the concept of a subabnormal subgroup $A$ of a group $G$, which is defined by a series of subgroups

$$A=A_0\subset A_1\subset\ldots\subset A_n=G,$$

where $A_i$ is abnormal in $A_{i+1}$, $i=0,\dots,n-1$.

#### References

 [1] B. Huppert, "Endliche Gruppen" , 1 , Springer (1967)

Nowadays, $A^g$ is mostly defined as $A^g=g^{-1}Ag$. Instead of "solvable" also "soluble" is frequently used.

How to Cite This Entry:
Abnormal subgroup. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Abnormal_subgroup&oldid=33149
This article was adapted from an original article by A.I. Kostrikin (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article