# Simple group

## Definition

QUICK PHRASES: prime number among groups, group without any proper nontrivial normal subgroup, group without any proper nontrivial quotients
No. Shorthand A group is simple if ... A group $G$ is simple if ...
1 Normal subgroup-based definition it is nontrivial and has no proper nontrivial normal subgroup $G$ is nontrivial and for any normal subgroup $H$ of $G$, either $H$ is trivial or $H = G$.
2 Surjective homomorphism-based definition it is nontrivial and any surjective homomorphism from it is either trivial or an isomorphism. $G$ is nontrivial and any surjective homomorphism $\varphi:G \to K$ is either trivial or an isomorphism.
3 Homomorphism-based definition it is nontrivial and any homomorphism of groups from it is either trivial or injective $G$ is nontrivial and given any homomorphism $\varphi:G \to K$, $\varphi$ is either injective (i.e., its kernel is trivial) or trivial (i.e., it maps everything to the identity element).
4 Subnormal subgroup-based definition it is nontrivial and has no proper nontrivial subnormal subgroup $G$ is nontrivial and for any subnormal subgroup $H$ of $G$, either $H$ is trivial or $H = G$.

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## Formalisms

### In terms of the simple group operator

This property is obtained by applying the simple group operator to the property: normal subgroup
View other properties obtained by applying the simple group operator

The group property of being simple is obtained by applying the simple group operator to the subgroup property of normality.

## Relation with other properties

This property is a pivotal (important) member of its property space. Its variations, opposites, and other properties related to it and defined using it are often studied

### Stronger properties

Property Meaning Proof of implication Proof of strictness (reverse implication failure) Intermediate notions
Absolutely simple group nontrivial, no proper nontrivial serial subgroup simple not implies absolutely simple |FULL LIST, MORE INFO
Strictly simple group nontrivial, no proper nontrivial ascendant subgroup simple not implies strictly simple |FULL LIST, MORE INFO
Simple non-abelian group simple and a non-abelian group |FULL LIST, MORE INFO
Finite simple group simple and a finite group Locally finite simple group|FULL LIST, MORE INFO
Finite simple non-abelian group finite, simple, and non-abelian |FULL LIST, MORE INFO

### Weaker properties

Property Meaning Proof of implication Proof of strictness (reverse implication failure) Intermediate notions
Characteristically simple group nontrivial, no proper nontrivial characteristic subgroup |FULL LIST, MORE INFO
Directly indecomposable group nontrivial, not expressible as an internal direct product of two proper subgroups Group having no proper nontrivial transitively normal subgroup, Monolithic group, Splitting-simple group, Subdirectly irreducible group|FULL LIST, MORE INFO
Centrally indecomposable group nontrivial, not expressible as a central product of two proper subgroups. In other words, every central factor is either the whole group or is contained in the center. Group having no proper nontrivial transitively normal subgroup|FULL LIST, MORE INFO
Splitting-simple group nontrivial, has no proper nontrivial complemented normal subgroup, i.e., is not expressible as an internal semidirect product of proper subgroups |FULL LIST, MORE INFO
Primitive group has a maximal subgroup that is also a core-free subgroup simple implies primitive primitive not implies simple |FULL LIST, MORE INFO
Monolithic group has a unique minimal normal subgroup contained in every nontrivial normal subgroup simple implies monolithic monolithic not implies simple |FULL LIST, MORE INFO
Group of finite composition length Group of composition length two, Group whose chief series are composition series|FULL LIST, MORE INFO
Normal-comparable group the lattice of normal subgroups is totally ordered |FULL LIST, MORE INFO
Group satisfying ascending chain condition on subnormal subgroups any ascending chain of subnormal subgroups stabilizes after a finite length |FULL LIST, MORE INFO
Group satisfying ascending chain condition on normal subgroups any ascending chain of normal subgroups stabilizes after a finite length Group of finite chief length, Group satisfying ascending chain condition on subnormal subgroups|FULL LIST, MORE INFO
Group in which every endomorphism is trivial or injective every endomorphism is either the trivial map or an injective endomorphism |FULL LIST, MORE INFO
Hopfian group every surjective endomorphism is an automorphism Group in which every endomorphism is trivial or injective, Group satisfying ascending chain condition on normal subgroups, Group satisfying ascending chain condition on subnormal subgroups|FULL LIST, MORE INFO

## Facts

### Proper subgroups are core-free

In a simple group, the normal core of any subgroup is a normal subgroup, and hence is either the whole group or the trivial subgroup. Thus, the normal core of any proper subgroup must be the trivial subgroup.

In other words, every proper subgroup is core-free.

### Nontrivial subgroups are contranormal

In a simple group, the normal closure of any subgroup is either the whole group or the trivial subgroup. Thus, the normal closure of any nontrivial subgroup is the whole group.

In other words, every nontrivial subgroup of a simple group is contranormal.

### Subgroup-defining functions collapse to trivial subgroup or whole group

Any subgroup-defining function (such as the center, the derived subgroup, the Frattini subgroup) returns a characteristic subgroup of the whole group. In other words, the center, derived subgroup, Frattini subgroup etc. are all characteristic subgroups.

Since every characteristic subgroup is normal, each of these is also a normal subgroup. But when the whole group is simple, this forces each of these to be either the trivial subgroup or the whole group. Thus, for instance:

### The only simple Abelian groups are cyclic groups of prime order

The proof of this follows more or less directly from the fact that in a simple Abelian group, every subgroup is normal, and hence, the subgroup generated by any nonidentity element is normal. This forces that the whole group is cyclic generated by any element, and hence it must be cyclic of prime order.

## Metaproperties

### Direct products

A direct product of simple groups is not simple. In fact, the two direct factors are themselves normal subgroups.

### Subgroups

Every finite group occurs as a subgroup of some simple group. Hence the property of being embeddable as a subgroup of a simple group is nothing distinguishing.

### Quotients

The only quotients of a simple group are itself and the trivial group.

## Testing

### The testing problem

Further information: Simplicity testing problem

### GAP command

This group property can be tested using built-in functionality of Groups, Algorithms, Programming (GAP).
The GAP command for this group property is:IsSimpleGroup
View GAP-testable group properties

To determine on GAP whether a given group is simple:

IsSimpleGroup (group)

where group could either be a definition of a group or a name for a group already defined.

## References

### Textbook references

• Abstract Algebra by David S. Dummit and Richard M. Foote, 10-digit ISBN 0471433349, 13-digit ISBN 978-0471433347, More info, Page 91
• A Course in the Theory of Groups by Derek J. S. Robinson, ISBN 0387944613, More info, Page 16
• Algebra by Michael Artin, ISBN 0130047635, 13-digit ISBN 978-0130047632, More info, Page 201, between points (2.3) and (2.4) (definition introduced in paragraph)