Minimal normal subgroup
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This article defines a subgroup property: a property that can be evaluated to true/false given a group and a subgroup thereof, invariant under subgroup equivalence. View a complete list of subgroup properties[SHOW MORE]
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 of a group is termed a minimal normal subgroup if it is normal and for any normal subgroup of such that , either or 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 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 , i.e. the cyclic group of order , there is a unique minimal normal subgroup of order for each prime divisor of .
- 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
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 such that each is normal in and is a minimal normal subgroup of .
Facts
General 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 and normal fully normalized implies minimal normal: In particular, any characteristically simple group is minimal normal in its holomorph. This is related to the fact that left transiter of normal is characteristic.
Facts in certain kinds of ambient groups
Additional conditions and conclusions
Statement | Additional condition | Conclusion |
---|---|---|
minimal normal subgroup with order not dividing index is characteristic, minimal normal subgroup with order greater than index is characteristic | the order of the subgroup does not divide its index | it is a characteristic subgroup |
normality-large and minimal normal implies monolith | normality-large subgroup: intersection with every nontrivial normal subgroup is normal | monolith, i.e., it is contained in every nontrivial normal subgroup, so the group is a monolithic group |
self-centralizing and minimal normal implies monolith (this follows from the previous fact and the fact that self-centralizing and normal implies normality-large) | self-centralizing subgroup: contains its own centralizer in the whole group | monolith, i.e., it is contained in every nontrivial normal subgroup, so the group is a monolithic group |
Other facts
- Minimal normal subgroup and core-free maximal subgroup need not be permutable complements
- Plinth theorem
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)