Minimal normal subgroup

This article is about a standard (though not very rudimentary) definition in group theory. The article text may, however, contain more than just the basic definition
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Definition

Symbol-free definition

A nontrivial subgroup of a group is termed a minimal normal subgroup if it is normal and the only normal subgroup properly contained inside it is the trivial subgroup.

Definition with symbols

A nontrivial subgroup $H$ of a group $G$ is termed a minimal normal subgroup if it is normal and for any normal subgroup $K$ of $G$ such that $K\le H$, either $K=H$ or $K$ is trivial.

Formalisms

In terms of the minimal operator

This property is obtained by applying the minimal operator to the property: nontrivial normal subgroup
View other properties obtained by applying the minimal operator

Examples

• In the group $\mathbb{Z}$ of integers, there are no minimal normal subgroups. That's because every number is a factor of some bigger number.
• In the group of integers mod $n$, i.e. the cyclic group of order $n$, there is a unique minimal normal subgroup of order $p$ for each prime divisor $p$ of $n$.
• In the symmetric group on three letters, the cyclic subgroup of order three (generated by a 3-cycle) is a minimal normal subgroup. The cyclic subgroup of order two is not a normal subgroup.
• In a nilpotent group, any minimal normal subgroup must actually be a minimal subgroup (i.e. it must be cyclic of prime order), that's because in a nilpotent group, any nontrivial normal subgroup intersects the center nontrivially.
• The subgroup comprising double transpositions and the identity, inside the symmetric group on four letters, is an example of a minimal normal subgroup that is not minimal as a subgroup (i.e., it contains proper nontrivial subgroups). This subgroup is abstractly isomorphic to the Klein four-group.

Relation with other properties

Stronger properties

• Simple normal subgroup
• Normal subgroup of prime order

Related notions

• Socle is the join of all minimal normal subgroups in a group. For a nilpotent p-group, the socle is Omega-1 of the center, see socle equals Omega-1 of center in nilpotent p-group.
• A monolithic group is a group that contains a unique minimal normal subgroup that is contained in every nontrivial normal subgroup. The minimal normal subgroup in this case is termed a monolith and it also coincides with the socle.
• Chief series is a normal series where each successive quotient is a minimal normal subgroup in the quotient of the whole group by the lower end. In other words, it is a series $\{e\}=H_0\le H_1\le \dots H_n=G$ such that each $H_i$ is normal in $G$ and $H_{i+1}/H_i$ is a minimal normal subgroup of $G/H_i$.

Facts

• Minimal normal implies characteristically simple: Any minimal normal subgroup must be a characteristically simple group, i.e., it must have no proper nontrivial characteristic subgroups. This follows from the fact that characteristic of normal implies normal.
• Characteristically simple implies minimal normal in holomorph: A characteristically simple group is a minimal normal subgroup in its holomorph. This is also related to the fact that left transiter of normal is characteristic.
• Minimal normal implies elementary abelian in finite solvable: A special case of the fact that minimal normal subgroups are characteristically simple, because the only characteristically simple subgroups in finite solvable groups are the elementary abelian ones.
• Minimal normal implies central in nilpotent, minimal normal implies contained in Omega-1 of center for nilpotent p-group: In nilpotent groups, minimal normal subgroups are cyclic of prime order and are contained in the center, and the converse is also true.

Testing

GAP command

This subgroup property can be tested using built-in functionality of Groups, Algorithms, Programming (GAP).
The GAP command for listing all subgroups with this property is:MinimalNormalSubgroups
View subgroup properties testable with built-in GAP command|View subgroup properties for which all subgroups can be listed with built-in GAP commands | View subgroup properties codable in GAP
Learn more about using GAP

References

Textbook references

• Finite Group Theory (Cambridge Studies in Advanced Mathematics) by Michael Aschbacher, ISBN 0521786754^{More info}, Page 25 (definition in paragraph, immediately suceeding definition of characteristically simple group)
• Groups and representations by Jonathan Lazare Alperin and Rowen B. Bell, ISBN 0387945261^{More info}, Page 93 (definition in paragraph)
• Abstract Algebra by David S. Dummit and Richard M. Foote, 10-digit ISBN 0471433349, 13-digit ISBN 978-0471433347^{More info}, Page 106 (no definition given; term implicitly introduced in exercises)