# Baer norm

This article is about a definition in group theory that is standard among the group theory community (or sub-community that dabbles in such things) but is not very basic or common for people outside.
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This article defines a subgroup-defining function, viz., a rule that takes a group and outputs a unique subgroup
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## Definition

### Symbol-free definition

The Baer norm of a group is defined in the following equivalent ways:

• It is the intersection of normalizers of all its subgroups
• It is the intersection of normalizers of all cyclic subgroups.
• It is the set of those elements of the group for which the corresponding conjugation is a power automorphism.

### Definition with symbols

The Baer norm of a group $G$ is defined as the intersection, over all subgroups $H$ of $G$ of the groups $N_G(H)$.

### In terms of the intersect-all operator

This property is obtained by applying the intersect-all operator to the property: normalizer subgroup
View other properties obtained by applying the intersect-all operator

## Group properties

The Baer norm is a Dedekind group, i.e., it is a group in which every subgroup is normal. Conversely, every Dedekind group equals its own Baer norm.

Further information: Baer norm is Dedekind

## Subgroup properties

### Properties satisfied

Property Meaning Proof of satisfaction
Normal subgroup
Hereditarily permutable subgroup every subgroup is a permutable subgroup of the whole group Baer norm is hereditarily permutable
Hereditarily 2-subnormal subgroup every subgroup is a 2-subnormal subgroup of the whole group Baer norm is hereditarily 2-subnormal
Characteristic subgroup invariant under all automorphisms Baer norm is characteristic
Strictly characteristic subgroup invariant under all surjective endomorphisms Baer norm is strictly characteristic

### Properties not satisfied

Property Meaning Proof of dissatisfaction
Fully invariant subgroup invariant under all endomorphisms Baer norm not is fully invariant
Hereditarily normal subgroup every subgroup is normal in the whole group Baer norm not is hereditarily normal

## Examples

### Dedekind groups

A Dedekind group is a group in which every subgroup is normal, or equivalently, a group that equals its own Baer norm. The finite Dedekind groups are precisely the following:

The smallest examples of Dedekind non-abelian groups are quaternion group and direct product of Q8 and Z2.

### Examples in groups of prime power order

Here are some examples where the Baer norm is a proper subgroup:

Group partSubgroup partQuotient part
Center of dihedral group:D8Dihedral group:D8Cyclic group:Z2Klein four-group

### Examples in other groups

Here are some examples in non-nilpotent groups:

## Relation with other subgroup-defining functions

### Smaller subgroup-defining functions

Subgroup-defining function Meaning Proof of containment Proof of strictness
Center Elements that commute with every element Baer norm contains center Center not contains Baer norm

### Larger subgroup-defining functions

Subgroup-defining function Meaning Proof of containment Proof of strictness
Wielandt subgroup intersection of normalizers of subnormal subgroup Wielandt subgroup contains Baer norm Baer norm not contains Wielandt subgroup
Second center second member of upper central series Second center contains Baer norm Baer norm not contains second center
Centralizer of derived subgroup centralizer of derived subgroup (commutator subgroup) Centralizer of derived subgroup contains Baer norm Baer norm not contains centralizer of derived subgroup

## Subgroup-defining function properties

### Reverse monotonicity

The Baer norm subgroup-defining function is weakly reverse monotone, that is, if $K$ is a subgroup of $G$ containing the Baer norm of $G$, then the Baer norm of $K$ contains the Baer norm of $H$.

### Idempotence and iteration

The Baer norm of a group equals its own Baer norm. A group equals its own Baer norm if and only if it is a Dedekind group, that is, every subgroup in it is normal.

### Quotient-idempotence and quotient-iteration

The quotient function corresponding to the Baer norm is not transitive.