Quasinormal subgroup

In mathematics, in the field of group theory, a quasinormal subgroup, or permutable subgroup, is a subgroup of a group that commutes (permutes) with every other subgroup. The term "quasinormal subgroup" was introduced by Oystein Ore in 1937.

Two subgroups are said to permute (or commute) if any element from the firstsubgroup, times an element of the second subgroup, can be written as an element of the secondsubgroup, times an element of the first subgroup. That is, $H$ and $K$as subgroups of $G$ are said to commute if "HK" = "KH", that is, any element of the form $hk$with $h\; in\; H$ and $k\; in\; K$ can be written in the form $k\text{'}h\text{'}$where $k\text{'}\; in\; K$ and $h\text{'}\; in\; H$.

Every quasinormal subgroup is a modular subgroup, that is, a modular element in the lattice of subgroups. This follows from the modular property of groups.

A conjugate permutable subgroup is one that commutes with all its conjugate subgroups. Every quasinormal subgroup is conjugate permutable.

Every normal subgroup is quasinormal, because, in fact, a normal subgroup commuteswith every element of the group. The converse is not true. For instance, any extension of a cyclic group of prime power order by another cyclic group of prime power order for the same prime, has the property that all its subgroups are quasinormal. However, not all of its subgroups need be normal.

Also, every quasinormalsubgroup of a finite group is a subnormal subgroup. This follows from the somewhatstronger statement that every conjugate permutable subgroup is subnormal, which in turnfollows from the statement that every maximal conjugate permutable subgroup is normal. (The finitenessis used crucially in the proofs.)

External links

* [http://www.maths.tcd.ie/pub/ims/bull56/GiG5612.pdf Old, Recent and New Results on Quasinormal subgroups]
* [http://sciences.aum.edu/~tfoguel/cp.pdf The proof that conjugate permutable subgroups are subnormal]