# Degree of irreducible representation divides index of abelian normal subgroup

*This article gives the statement, and possibly proof, of a constraint on numerical invariants that can be associated with a finite group*

This article states a result of the form that one natural number divides another. Specifically, the (degree of a linear representation) of a/an/the (irreducible linear representation) divides the (index of a subgroup) of a/an/the (abelian normal subgroup).

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This fact is related to: linear representation theory

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## Statement

### For an algebraically closed field of characteristic zero

Let be a finite group and an irreducible representation of over an algebraically closed field of characteristic zero. Let be an Abelian normal subgroup (?) of . Then the degree of divides the index .

### For a splitting field of characteristic zero

Let be a finite group and an irreducible representation of over a splitting field for of characteristic zero. Let be an Abelian normal subgroup (?) of . Then the degree of divides the index .

Note that the two statements are equivalent because any representation of that is irreducible over a splitting field is also irreducible over the algebraic closure.

### For a splitting field, any characteristic not dividing the group order

Let be a finite group and an irreducible representation of over a splitting field for of any characteristic (which must not divide the order of ). Let be an Abelian normal subgroup (?) of . Then the degree of divides the index .

This is equivalent to the other two formulations because degrees of irreducible representations are the same for all splitting fields.

## Related facts

### Similar divisibility facts

`Further information: degrees of irreducible representations`

- Degree of irreducible representation divides group order
- Degree of irreducible representation divides index of center (or equivalently, it divides the order of the inner automorphism group)
- Degree of irreducible representation divides index of abelian subgroup in finite nilpotent group
- Square of degree of irreducible representation divides order of inner automorphism group in finite nilpotent group

### Combinatorial/inequality facts

- Degree of irreducible representation is bounded by index of abelian subgroup
- Order of inner automorphism group bounds square of degree of irreducible representation
- Sum of squares of degrees of irreducible representations equals order of group

### Converse

### Opposite facts

### Applications

- Ito-Michler theorem: The easy direction of the theorem follows immediately from this statement. It basically states that if a finite group has an abelian normal -Sylow subgroup, then does not divide the degrees of any of its irreducible representations. The converse, which is the hard part of the Ito-Michler theorem, relies on much heavier machinery including the classification of finite simple groups.

### Breakdown for a field that is not algebraically closed

`Further information: cyclic group:Z3, linear representation theory of cyclic group:Z3`

Let be the cyclic group of order three and (real numbers) be the field. Then, there are two irreducible representations of over : the trivial representation, and a two-dimensional representation given by the action by rotation by multiples of . The two-dimensional representation has degree , and this does *not* divide the order of the group, which is .

This representation does *not* remain irreducible over : in fact, it decomposes as a direct sum of two irreducible representations, both of which have degree , which satisfies the condition of dividing the index of any abelian normal subgroup.

## Examples

Note that the result is true for trivial reasons for a finite abelian group. We thus concentrate on non-abelian groups. In the table below, we list, for each group, the indexes of all subgroups that are maximal among abelian normal subgroups, the gcd of these indexes, the degrees of irreducible representations, and the lcm of these degrees. By the result stated, the lcm of degree of irreducible representations must divide the gcd of indices of all subgroups that are maximal among abelian normal subgroups. Note that in fact we do not need to take gcd -- we can take the *minimum*, because every finite group has an abelian normal subgroup whose order is divisible by the orders of all abelian normal subgroups.

Note that among the examples below, special linear group:SL(2,3) is the only case where the lcm of degrees of irreducible representations is strictly smaller than the gcd of index values for abelian normal subgroups.

Group | Linear representation theory | Indexes of maximal among abelian normal subgroups | gcd of index values | Degrees of irreducible representations | lcm of degrees |
---|---|---|---|---|---|

symmetric group:S3 | linear representation theory of symmetric group:S3 | 2 | 2 | 1,1,2 | 2 |

dihedral group:D8 | linear representation theory of dihedral group:D8 | 2,2,2 | 2 | 1,1,1,1,2 | 2 |

quaternion group | linear representation theory of quaternion group | 2,2,2 | 2 | 1,1,1,1,2 | 2 |

dihedral group:D10 | linear representation theory of dihedral group:D10 | 2 | 2 | 1,1,2,2 | 2 |

alternating group:A4 | linear representation theory of alternating group:A4 | 3 | 3 | 1,1,1,3 | 3 |

symmetric group:S4 | linear representation theory of symmetric group:S4 | 6 | 6 | 1,1,2,3,3 | 6 |

special linear group:SL(2,3) | linear representation theory of special linear group:SL(2,3) | 12 | 12 | 1,1,1,2,2,2,3 | 6 |

alternating group:A5 | linear representation theory of alternating group:A5 | 60 | 60 | 1,3,3,4,5 | 60 |

## Facts used

- Third isomorphism theorem
- Index is multiplicative
- Normality satisfies image condition
- Abelianness is quotient-closed
- Isotypical-or-induced lemma: Let be a finite group, a normal subgroup, and an algebraically closed field of characteristic zero (remove those assumptions on ?). Then if is an irreducible representation of one of the following must hold:
- is induced from an irreducible representation of , where is a proper subgroup of containing
- The restriction of to is an isotypical representation

- Normality satisfies intermediate subgroup condition
- degree of irreducible representation divides index of center: This result states that the degree of any irreducible representation must divide the index of the center, or equivalently, the order of the inner automorphism group.

## Proof

This proof uses a tabular format for presentation. Provide feedback on tabular proof formats in a survey (opens in new window/tab) | Learn more about tabular proof formats|View all pages on facts with proofs in tabular format

This proof uses the principle of mathematical induction in a nontrivial way (i.e., it would be hard to write the proof clearly without explicitly using induction).

We prove the statement by a strong form of induction on the order.

**Statement to be proved by induction** on : For any finite group of order , and any abelian normal subgroup of the group, and any algebraically closed field of characteristic zero, the degree of any irreducible linear representation of the finite group over must divide the index of the abelian normal subgroup.

Our strong form uses that the statement is true for all smaller numbers.

**Inductive hypothesis**: The statement to be proved by induction is true for all . In particular, it is true for all proper divisors of .

**Inductive goal**:

**Given**: A group of order . An abelian normal subgroup of . An irreducible linear representation of over an algebraically closed field of characteristic zero.

**To prove**: The degree of divides the index .

**Proof**: Suppose has kernel . Let .

Step no. | Assertion/construction | Facts used | Given data used | Previous steps used | Explanation |
---|---|---|---|---|---|

1 | has an abelian normal subgroup whose index divides the index . | Facts (1), (2) | is abelian normal in . | [SHOW MORE] | |

2 | descends to an irreducible linear representation of , having the same degree as . | is irreducible, has kernel | [SHOW MORE] | ||

3 | If is not faithful, i.e., is nontrivial, then the degree of divides . | Steps (1), (2) | [SHOW MORE] | ||

4 | Either is induced from an irreducible representation of , where is a proper subgroup of containing , or the restriction of to is isotypical. | Fact (5) | is irreducible over an algebraically closed field (or splitting field) of characteristic zero, is finite, and is normal in . | Fact-direct | |

5 | If is induced from an irreducible representation of , where is a proper subgroup of containing , then | Definition of induced representation | |||

6 | If is induced from an irreducible representation of , we must have that divides . | Fact (6) | Step (5) (for notation) | [SHOW MORE] | |

7 | If is induced from an irreducible representation of , we obtain that the degree of divides . | Fact (2) | Steps (5), (6) | [SHOW MORE] | |

8 | If is faithful and the restriction of to is isotypical, then is contained in the center of . | is abelian. | [SHOW MORE] | ||

9 | If is faithful and the restriction of to is isotypical, then the degree of divides . | Facts (2),(7) | Step (8) | [SHOW MORE] | |

10 | In every case, the degree of divides . | Steps (3), (4), (7), (9) | [SHOW MORE] |