# Hall 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

### Definition without prime set specification

A subgroup $H$ of a finite group $G$ is termed a Hall subgroup if it satisfies the following equivalent conditions:

• The order of $H$ is relatively prime to the index of $H$ in $G$.
• For any prime number $p$ dividing the order of $G$, $p$ divides exactly one of the two numbers: the order of $H$ and the index of $H$ in $G$.

### Definition with prime set specification

Suppose $\pi$ is a set of prime numbers and $G$ is a finite group. A subgroup $H$ of $G$ is termed a $\pi$-Hall subgroup or Hall $\pi$-subgroup if it satisfies the following equivalent conditions:

1. All the primes dividing the order of $H$ are in the prime set $\pi$ and all the primes dividing the index of $H$ in $G$ are outside the prime set $\pi$.
2. The order of $G$ is the unique largest divisor of the order of $G$ that has the property that all its prime divisors are in $\pi$. In other words, it is the $\pi$-part of the order of $G$.

We sometimes use the notation $\pi'$ to refer to the complement of $\pi$ in the set of prime numbers.

### Relation between order and prime set specification

• The order of a Hall $\pi$-subgroup of $G$ depends only on the prime set $\pi$ and on the order of $G$. In particular, for fixed $\pi$, all Hall $\pi$-subgroups have the same order.
• Conversely, if two Hall subgroups of a group have the same order, then the prime set specifications that work for one Hall subgroup also work for the other.
• As far as the definition of Hall $\pi$-subgroup of $G$ is concerned, we only care about the intersection of $\pi$ with the set of prime divisors of the order of $G$. Adding or removing primes that do not divide the order of $G$ does not affect the notion of Hall $\pi$-subgroup.

## Examples

### Extreme examples

• The trivial subgroup is a Hall subgroup in any finite group. [SHOW MORE]
• Every finite group is a Hall subgroup of itself. [SHOW MORE]

### Sylow subgroups and p-complements

There are two other important near-extremes of Hall subgroups:

• Sylow subgroups are Hall subgroups corresponding to a single prime. In other words, a Sylow subgroup is a finite p-group whose index is relatively prime to $p$. If $p$ divides the order of the group, $p$-Sylow subgroups must be nontrivial. Sylow's theorem guarantees the existence and other nice behavior of the $p$-Sylow subgroup for any prime $p$ in any finite group.
• p-complements are Hall subgroups whose index is a prime power. In other words, they are Hall subgroups whose prime set excludes at most one prime divisor of the order of the group. A $p$-complement is thus a Hall $p'$-subgroup where $p'$ is the set of primes other than $p$. (As always, we only care about the primes that divide the order of the group).

### Particular examples

• A3 in S3: The subgroup has order 3 and index 2 in a group of order 6. It is a 3-Sylow subgroup and also a 2-complement.
• A4 in A5: The subgroup has order 12 and index 5 in a group of order 60. It is a $\{ 2,3 \}$-Hall subgroup and also a 5-complement.
• S4 in S5: The subgroup has order 24 and index 5 in a group of order 120. It is a $\{ 2,3 \}$-Hall subgroup and also a 5-complement.

Here is a list of examples:

Group partSubgroup partQuotient part
A3 in S3Symmetric group:S3Cyclic group:Z3Cyclic group:Z2
A4 in A5Alternating group:A5Alternating group:A4
S2 in S3Symmetric group:S3Cyclic group:Z2
S4 in S5Symmetric group:S5Symmetric group:S4

## Facts

### Existence and domination

• [[Existence of pi-subgroups for all prime sets $\pi$ is equivalent to existence of p-complements for all primes p]]
• [[ECD condition for $\pi$-subgroups in finite solvable groups]]: This states that in finite solvable groups, $\pi$-Hall subgroup exist for all prime sets $\pi$, they are conjugate, and they dominate $\pi$-subgroups.
• Hall's theorem: This is a converse to the above, stating that if $\pi$-Hall subgroups exist for all prime sets $\pi$, then the group is solvable.

### General non-existence and other results

• Hall subgroups need not exist
• Hall not implies order-isomorphic: It is possible for two $\pi$-Hall subgroups of a finite group $G$ to be non-isomorphic.
• Hall not implies isomorph-conjugate: It is possible for two isomorphic $\pi$-Hall subgroups of a finite group $G$ to not be conjugate subgroups inside $G$.

## Relation with other properties

### Stronger properties

Property Meaning Proof of implication Proof of strictness (reverse implication failure) Intermediate notions
Sylow subgroup Hall subgroup for a single prime, i.e., finite p-group whose index is relatively prime to $p$ for some prime $p$ Nilpotent Hall subgroup, Pronormal Hall subgroup|FULL LIST, MORE INFO
p-complement Hall $p'$-subgroup, i.e., Hall subgroup whose index is a prime power |FULL LIST, MORE INFO
normal Hall subgroup Hall subgroup that is also a normal subgroup Pronormal Hall subgroup|FULL LIST, MORE INFO
Normal Sylow subgroup Sylow subgroup that is also a normal subgroup Sylow subgroup|FULL LIST, MORE INFO
Hall retract Hall subgroup that is also a retract, i.e., it has a normal complement. Note that the normal complement must also be a Hall subgroup for the complementary set of primes Conjugacy-closed Hall subgroup, Pronormal Hall subgroup|FULL LIST, MORE INFO
Sylow retract Sylow subgroup that is also a retract, i.e., $p$-Sylow subgroup in a group that has a normal p-complement Conjugacy-closed Hall subgroup, Sylow subgroup|FULL LIST, MORE INFO
nilpotent Hall subgroup Hall subgroup that is also a nilpotent group Pronormal Hall subgroup|FULL LIST, MORE INFO
order-dominating Hall subgroup Hall subgroup that is also an order-dominating subgroup, i.e., any subgroup of the whole group whose order divides it is conjugate to a subgroup of it Hall not implies order-dominating |FULL LIST, MORE INFO
order-conjugate Hall subgroup Hall subgroup that is also an order-conjugate subgroup, i.e., all Hall subgroups of that order are conjugate subgroups Hall not implies order-conjugate |FULL LIST, MORE INFO
isomorph-conjugate Hall subgroup Hall subgroup that is also an isomorph-conjugate subgroup, i.e., it is conjugate to all isomorphic subgroups Hall not implies isomorph-conjugate |FULL LIST, MORE INFO
pronormal Hall subgroup Hall subgroup that is also a pronormal subgroup |FULL LIST, MORE INFO

### Weaker properties

Property Meaning Proof of implication Proof of strictness (reverse implication failure) Intermediate notions
join of Sylow subgroups join of Sylow subgroups of the whole group Hall implies join of Sylow subgroups |FULL LIST, MORE INFO
join of automorph-conjugate subgroups join of automorph-conjugate subgroups of the whole group (via join of Sylow sugroups) Join of Sylow subgroups|FULL LIST, MORE INFO
core-characteristic subgroup normal core is a characteristic subgroup |FULL LIST, MORE INFO
closure-characteristic subgroup normal closure is a characteristic subgroup Join of Sylow subgroups, Join of automorph-conjugate subgroups, Subgroup whose normal closure is fully invariant, Subgroup whose normal closure is homomorph-containing|FULL LIST, MORE INFO
paracharacteristic subgroup contranormal in its join with any automorphic subgroup (via join of Sylow subgroups, see also Hall implies paracharacteristic) Join of Sylow subgroups|FULL LIST, MORE INFO
paranormal subgroup contranormal in its join with any conjugate subgroup (via paracharacteristic) Join of Sylow subgroups|FULL LIST, MORE INFO
polycharacteristic subgroup (via paracharacteristic) Join of Sylow subgroups|FULL LIST, MORE INFO
polynormal subgroup (via paranoral, also via polycharacteristic) Join of Sylow subgroups, Paranormal subgroup|FULL LIST, MORE INFO
intermediately normal-to-characteristic subgroup for any intermediate subgroup in which it is normal, it is also characteristic in that subgroup |FULL LIST, MORE INFO
intermediately subnormal-to-normal subgroup for any intermediate subgroup in which it is subnormal, it is also normal in that subgroup Intermediately normal-to-characteristic subgroup, Paranormal subgroup|FULL LIST, MORE INFO

## Metaproperties

### Transitivity

This subgroup property is transitive: a subgroup with this property in a subgroup with this property, also has this property in the whole group.
ABOUT THIS PROPERTY: View variations of this property that are transitive | View variations of this property that are not transitive
ABOUT TRANSITIVITY: View a complete list of transitive subgroup properties|View a complete list of facts related to transitivity of subgroup properties |Read a survey article on proving transitivity

A Hall subgroup of a Hall subgroup is a Hall subgroup. This follows from the fact that the index is multiplicative. For full proof, refer: Hall satisfies transitivity

### Trimness

This subgroup property is trim -- it is both trivially true (true for the trivial subgroup) and identity-true (true for a group as a subgroup of itself).
View other trim subgroup properties | View other trivially true subgroup properties | View other identity-true subgroup properties

The property of being a Hall subgroup is trivially true, that is, the trivial subgroup is a Hall subgroup in any group.

It is also identity-true, that is, every finite group is a Hall subgroup of itself.

### Intermediate subgroup condition

YES: This subgroup property satisfies the intermediate subgroup condition: if a subgroup has the property in the whole group, it has the property in every intermediate subgroup.
ABOUT THIS PROPERTY: View variations of this property satisfying intermediate subgroup condition | View variations of this property not satisfying intermediate subgroup condition
ABOUT INTERMEDIATE SUBROUP CONDITION:View all properties satisfying intermediate subgroup condition | View facts about intermediate subgroup condition

This states that if $H$ is a Hall subgroup of $G$ and $K$ is some subgroup containing $H$, then $H$ is a Hall subgroup of $K$.

For full proof, refer: Hall satisfies intermediate subgroup condition

### Transfer condition

This subgroup property does not satisfy the transfer condition

For full proof, refer: Hall does not satisfy transfer condition

## History

The notion of Hall subgroup was introduced by Philip Hall who studied their properties and proved the theorem that a group is solvable if and only if it has Hall subgroups of all possible orders. (see ECD condition for pi-subgroups in finite solvable groups and Hall's theorem).