# Malnormal subgroup

In mathematics, in the field of group theory, a subgroup ${\displaystyle H}$ of a group ${\displaystyle G}$ is termed malnormal if for any ${\displaystyle x}$ in ${\displaystyle G}$ but not in ${\displaystyle H}$, ${\displaystyle H}$ and ${\displaystyle xHx^{-1}}$ intersect in the identity element.[1]

Some facts about malnormality:

• An intersection of malnormal subgroups is malnormal.[2]
• Malnormality is transitive, that is, a malnormal subgroup of a malnormal subgroup is malnormal.[3]
• The trivial subgroup and the whole group are malnormal subgroups. A normal subgroup that is also malnormal must be one of these.[4]
• Every malnormal subgroup is a special type of C-group called a trivial intersection subgroup or TI subgroup.

When G is finite, a malnormal subgroup H distinct from 1 and G is called a "Frobenius complement".[4] The set N of elements of G which are, either equal to 1, or non-conjugate to any element of H, is a normal subgroup of G, called the "Frobenius kernel", and G is the semi-direct product of H and N (Frobenius' theorem).[5]

## References

1. ^ Lyndon, Roger C.; Schupp, Paul E. (2001), Combinatorial Group Theory, Springer, p. 203, ISBN 9783540411581.
2. ^ Gildenhuys, D.; Kharlampovich, O.; Myasnikov, A. (1995), "CSA-groups and separated free constructions", Bulletin of the Australian Mathematical Society, 52 (1): 63–84, MR 1344261, arXiv:, doi:10.1017/S0004972700014453.
3. ^ Karrass, A.; Solitar, D. (1971), "The free product of two groups with a malnormal amalgamated subgroup", Canadian Journal of Mathematics, 23: 933–959, MR 0314992, doi:10.4153/cjm-1971-102-8.
4. ^ a b de la Harpe, Pierre; Weber, Claude (2011), Malnormal subgroups and Frobenius groups: basics and examples, arXiv:.
5. ^ Feit, Walter (1967), Characters of finite groups, W. A. Benjamin, Inc., New York-Amsterdam, pp. 133–139, MR 0219636.