Malnormal subgroup

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In mathematics, in the field of group theory, a subgroup of a group is termed malnormal if for any in but not in , and 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]


  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. ^ a b 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.