# Frattini subgroup

This article defines a subgroup-defining function, viz., a rule that takes a group and outputs a unique subgroup
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## Definition

QUICK PHRASES: intersection of all maximal subgroups, biggest subgroup contained in every maximal subgroup

Let $G$ be a group. The Frattini subgroup of $G$, denoted $\Phi(G)$, is defined in the following equivalent ways:

1. It is the intersection of all subgroups $M \le G$, where $M$ is maximal in $G$
2. It is the set of all nongenerators, i.e., elements $x \in G$ such that if $S \cup \{ x \}$ is a generating set for $G$, then so is $S$

When $G$ is a group in which every subgroup is contained in a maximal subgroup, then the Frattini subgroup is also the unique largest Frattini-embedded normal subgroup.

### Equivalence of definitions

Further information: Equivalence of definitions of Frattini subgroup

## Effect of operators

### Fixed-point operator

A group equals its own Frattini subgroup if and only if it has no maximal subgroups. For instance, the group of rational numbers, or more generally, the additive group of any field of characteristic zero has no maximal subgroups.

### Free operator

A group whose Frattini subgroup is trivial is termed a Frattini-free group.

## Group properties satisfied

In general, it is hard to find group properties satisfied by the Frattini subgroup of every group. However, for a group in which every subgroup is contained in a maximal subgroup, the Frattini subgroup usually satisfies some fairly strong restrictions. Most of these are restrictions that are satisfied by any Frattini-embedded normal subgroup. These include:

Since any finite ACIC-group is nilpotent, the Frattini subgroup of any finite group is nilpotent. For full proof, refer: Frattini subgroup is nilpotent

Also related are:

## Subgroup properties satisfied

The Frattini subgroup of any group is a characteristic subgroup.

For a finite group, the Frattini subgroup is always a proper subgroup (because there exist maximal subgroups).

## Subgroup-defining function properties

### Monotonicity

The Frattini subgroup-defining function is not monotone. However, the Frattini subgroup of any normal subgroup is contained in the Frattini subgroup of the whole group, provided that normal subgroup satisfies the property that every proper subgroup is contained in a maximal subgroup. In particular, for any finite group, the Frattini subgroup of a normal subgroup is contained in the Frattini subgroup of the whole group.

For full proof, refer: Frattini subgroup is normal-monotone

### Idempotence and iteration

The Frattini subgroup-defining function is not idempotent.

### Quotient-idempotence

This subgroup-defining function is quotient-idempotent: taking the quotient of any group by the subgroup, gives a group where the subgroup-defining function yields the trivial subgroup
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## Associated constructions

### Associated quotient-defining function

The quotient-defining function associated with this subgroup-defining function is: Frattini quotient

### Associated descending series

The associated descending series to this subgroup-defining function is: Frattini series

The Frattini series is the series obtained by iterating the Frattini subgroup operation, starting with the whole group. It gives aa descending series.

If this series terminates at the identity in finite length (which it will for a finite group, since the Frattini subgroup at each stage will be proper) then the length of the series is termed the Frattini length for the group.

## Computation

### GAP command

The command for computing this subgroup-defining function in Groups, Algorithms and Programming (GAP) is:FrattiniSubgroup
View other GAP-computable subgroup-defining functions