Varying normality

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Normality is one of the most pivotal subgroup properties. It traces its origins to the very beginnings of group theory, in fact, to even before that. Given its long history and the varied ways in which it turns up, it is natural that a large number of variations of normality have popped up in group theory.

This article surveys some of the more common among the many variations of the subgroup property of normality, trying to organize them into themes and streams. There are three basic ideas behind variation:

• Emulate the strengths
• Remedy the weaknesses
• Weaken or remove the strengths

Finding right transiters

Lack of transitivity is one of the major problems with normality. One way of remedying this problem is to find transitive subgroup properties $q$ such that every normal subgroup of a subgroup with property $q$ is normal.

All the properties $q$ discussed below satisfy the following:

• Every normal subgroup of a subgroup with property $q$ is also normal
• The subgroup property is transitive
• The subgroup property satisfies intermediate subgroup condition

Transitively normal subgroup

Further information: transitively normal subgroup

The property of being a transitively normal subgroup is the right transiter for the subgroup property of normality. It is defined as follows:

$H$ is transitively normal in $G$ if whenever $K$ is a normal subgroup of $H$, then $K$ is also normal in $G$.

Alternatively, observe that the property of being a normal subgroup can be expressed in the function restriction formalism as:

Quotientable automorphism $\to$ Automorphism

This is a left tight restriction formal expression, and hence the right transiter of normality is:

Quotientable automorphism $\to$ Quotientable automorphism

This is the same as the proeprty of being transitively normal.

Conjugacy-closed normal subgroup

Further information: conjugacy-closed normal subgroup

The property of being a conjugacy-closed normal subgroup is equivalent to the property of being both normal, and conjugacy-closed. A subgroup is termed conjugacy-closed if any two elements in the subgroup that are conjugate in the whole group are also conjugate in the subgroup.

Alternatively, we can view the property of being conjugacy-closed normal as follows.

The property of being normal can be written as:

Class automorphism $\to$ Automorphism

Hence the property:

Class automorphism $\to$ Class automorphism

is stronger than the right transiter of normality. This property is precisely the same as the property of being a conjugacy-closed normal subgroup.

Central factor

Further information: central factor The property of being a central factor is defined as follows: $H$ is a central factor of $G$ if $HC_{G}(H)=G$. Equivalently, observe that the subgroup property of normality can be expressed as:

Inner automorphism $\to$ Automorphism

Thus, the following property is clearly stronger than the right transiter:

Inner automorphism $\to$ Inner automorphism

This is precisely the same as the property of being a central factor.

Direct factor

Further information: direct factor

We know that the subgroup property of being a direct factor is a t.i. subgroup property, and that it satisfies the intermediate subgroup condition. Further, a direct factor is clearly a central factor, hence it is stronger than the right transiter of normality.

Finding left transiters

Characteristic subgroup

Further information: characteristic subgroup The property of normality can be expressed as:

Inner automorphism $\to$ Automorphism

Further, this expression is right tight for normality, hence the left transiter of normality is the property:

Automorphism $\to$ Automorphism

Which is the property of being a characteristic subgroup.

Obtaining a handle on the quotient

When we have a normal subgroup, there's a natural quotient group. Two questions arise:

• What can we say about the quotient as an abstract group?
• To what extent can we realize the quotient as a subgroup?

Direct factor

Further information: direct factor

A direct factor is a normal subgroup that has a complement which is also a normal subgroup. In other words, $H$ is a direct factor in $G$ if there is a subgroup $K$ of $G$ such that $H$ and $K$ are both normal, $H\cap K$ is trivial, and $HK=G$.

Notice that if $H$ is a direct factor of $G$, the quotient group $G/H$ is isomorphic to $K$ in a natural map -- the map that sends each coset of $H$ in $G$ to the unique element of $K$ in that coset.

Complemented normal subgroup

Further information: complemented normal subgroup

A complemented normal subgroup is a normal subgroup $H$ such that there exists a subgroup $K$ of $G$ such that $H\cap K$ is trivial and $HK=G$. We no longer assume that $K$ is also normal.

The quotient group $G/H$ is isomorphic to $K$ via the map that sends each coset of $H$ to the unique element of $K$ that lies inside that coset.

This is still somewhat nice: it means that the quotient occurs as a subgroup in a sufficiently natural way.

Related notions are the notion of retract (in this setup $K$ is a retract and the quotient map from $G$ to $K$ is a retraction) and the notion of semidirect product (here $G$ is the internal semidirect product of $H$ by $K$).

Regular kernel

Further information: regular kernel

Endomorphic kernel

Further information: endomorphic kernel

A somewhat weaker requirement than being able to find a complement to the normal subgroup is being able to find a subgroup that is isomorphic as an abstract group to the quotient.

A normal subgroup is termed an endomorphic kernel if it occurs as the kernel of an endomorphism from the group, or equivalently, if there is a subgroup of the group isomorphic as an abstract group to the quotient by this normal subgroup.