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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 only in the identity element.

Some facts about malnormality:

• An intersection of malnormal subgroups is malnormal.
• Malnormality is transitive, that is, a malnormal subgroup of a malnormal subgroup is malnormal.
• The trivial subgroup and the whole group are malnormal subgroups. A normal subgroup that is also malnormal must be one of these.
• 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". 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).

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, arXiv:math/9605203, doi:10.1017/S0004972700014453, MR 1344261.
3. Karrass, A.; Solitar, D. (1971), "The free product of two groups with a malnormal amalgamated subgroup", Canadian Journal of Mathematics, 23: 933–959, doi:10.4153/cjm-1971-102-8, MR 0314992.
4. ^ de la Harpe, Pierre; Weber, Claude (2011), Malnormal subgroups and Frobenius groups: basics and examples, arXiv:1104.3065, Bibcode:2011arXiv1104.3065D.
5. Feit, Walter (1967), Characters of finite groups, W. A. Benjamin, Inc., New York-Amsterdam, pp. 133–139, MR 0219636.

• Subgroup properties