# Fitting subgroup

This article defines a subgroup-defining function, viz., a rule that takes a group and outputs a unique subgroup
View a complete list of subgroup-defining functions OR View a complete list of quotient-defining functions

## History

### Origin

The notion of Fitting subgroup was introduced by Hans Fitting.

## Definition

### Symbol-free definition

The Fitting subgroup of a group is defined as the subgroup generated by all its nilpotent normal subgroups.

For a finite group, it can be defined in the following equivalent ways:

1. It is the unique largest nilpotent characteristic subgroup, and hence, also the join of all nilpotent characteristic subgroups.
2. It is the unique largest nilpotent normal subgroup, and hence, also the join of all nilpotent normal subgroups.
3. it is the unique largest nilpotent subnormal subgroup, and hence, also the join of all nilpotent subnormal subgroups.

### Definition with symbols

The Fitting subgroup of a group $G$, denoted as $F(G)$, is defined as the subgroup generated by all nilpotent normal subgroups of $G$. In the particular case where $G$ is finite, the Fitting subgroup is itself a nilpotent group and can be defined in the following equivalent ways:

1. $F(G)$ is the unique largest nilpotent characteristic subgroup of $G$, and hence, also the join of all nilpotent characteristic subgroups of $G$.
2. $F(G)$ is the unique largest nilpotent normal subgroup of $G$, and hence, also the join of all nilpotent normal subgroups of $G$.
3. $F(G)$ is the unique largest nilpotent subnormal subgroup of $G$, and hence, also the join of all nilpotent subnormal subgroups of $G$.

### For a finite group

For a finite group, the Fitting subgroup is the direct product of $p$-cores for all the primes $p$.

## Group properties satisfied

The Fitting subgroup of any group is a Fitting group, viz a group generated by normal nilpotent subgroups. For a finite group, the Fitting subgroup is a nilpotent group.

### In terms of the group property core operator

This property is obtained by applying the group property core operator to the property: nilpotent group
View other properties obtained by applying the group property core operator

## Effect of operators

### Free operator

A group whose Fitting subgroup is trivial is termed a Fitting-free group. A group is Fitting-free if and only if it has no proper nontrivial normal Abelian subgroups.

### Fixed-point operator

A group is its own Fitting subgroup if and only if it is a Fitting group. For finite groups, this is equivalent to the condition of being a nilpotent group.

## Subgroup-defining function properties

### Idempotence

This subgroup-defining function is idempotent. In other words, applying this twice to a given group has the same effect as applying it once

The Fitting subgroup of the Fitting subgroup is the Fitting subgroup. The fixed points are precisely the Fitting groups.

## Associated constructions

### Associated quotient-defining function

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

### Associated ascending series

The associated ascending series to this subgroup-defining function is: Fitting series

The ascending series wherein each successive quotient is the Fitting subgroup of the quotient by the lower term of the whole group, is termed the Fitting series. For finite groups, the Fitting series terminates in finitely many steps at the whole group if and only if the group is solvable.

The length of the Fitting series for a given group is termed its Fitting length.

## Computation

### GAP command

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