# Extensible automorphisms problem

*This article describes an open problem in the following area of/related to group theory*: group theory

## Contents

## Statement

Suppose is a group property. The **extensible automorphisms problem** for is the problem of determining all the group property-conditionally extensible automorphisms with respect to property . An automorphism of a group satisfying is extensible conditional to if for any group containing and satisfying , extends to an automorphism of .

Variants involve:

- requiring that the automorphism extend not just once but repeatedly
- replacing extensible automorphism by pushforwardable automorphism, quotient-pullbackable automorphism, extensible endomorphism, or some other closely related notion, and
- replacing automorphism by endomorphism, local isomorphism or some other weaker notion.

## The three main formulations and their resolutions for different group properties

### Extensible, pushforwardable, and quotient-pullbackable automorphisms

Suppose is a group property, a group satisfying , and an automorphism of . We say that is:

- extensible with respect to if for any group containing and satisfying , there is an automorphism of whose restriction to equals .
- pushforwardable with respect to if for any group satisfying and homomorphism , there exists an automorphism of such that .
- quotient-pullbackable with respect to if for any group satisfying and surjective homomorphism , there exists an automorphism of such that .

An inner automorphism of a group satisfies all these properties.

### Known results

Note that since extensible implies pushforwardable for any property, a *yes* for extensible implies a *yes* for pushforwardable.

Group property | Extensible equals inner? | Pushforwardable equals inner? | Quotient-pullbackable equals inner? |
---|---|---|---|

any group | yes | yes | yes |

finite group | yes | yes | yes |

group of prime power order (fixed prime) | yes | yes | yes |

solvable group | yes | yes | yes |

periodic group | yes | yes | yes |

p-group | yes | yes | yes |

finite solvable group | yes | yes | yes |

pi-group | yes | yes | yes |

finite pi-group | yes | yes | yes |

abelian group | no (see abelian-extensible automorphism) | no | no (see abelian-quotient-pullbackable automorphism) |

finite abelian group | no (e.g., inverse map) | no | no (e.g., inverse map) |

group of nilpotency class two | ? | ? | ? |

## Variations with conditions on subgroup embeddings

- Normal-extensible automorphisms problem: This problem seeks to characterize all the normal-extensible automorphisms of a group. A normal-extensible automorphism of a group is an automorphism that can always be extended to a bigger group containing the group as a normal subgroup. Normal-extensible automorphisms of a group need
*not*be inner. In fact, they need not even preserve normal subgroups. - Characteristic-extensible automorphisms problem: This problem seeks to characterize all the characteristic-extensible automorphisms of a group. These need
*not*be inner.

### Replacing automorphisms by other kinds of maps

- Extensible local isomorphisms theorem: This states that any extensible local isomorphism, i.e., any isomorphism between subgroups that can always be extended to an automorphism for any bigger group must in fact extend to an inner automorphism of the given group. We can also consider problems of normal-extensible local isomorphisms and characteristic-extensible local isomorphisms.
- Extensible endomorphisms problem: This problem seeks to classify the extensible endomorphisms, i.e., the endomorphisms of a group that can be extended to endomorphisms for any bigger group containing it. There are also corresponding notions of pushforwardable endomorphism, quotient-pullbackable endomorphism. We can also consider problems of normal-extensible endomorphisms and characteristic-extensible endomorphisms.

### Extensible automorphisms problem on subvarieties of the variety of groups

`Further information: Variety-extensible automorphisms problem, Quasivariety-extensible automorphisms problem`

Let be a variety of algebras and be an algebra in . An automorphism of is termed -extensible, or variety-extensible for the variety , if for any algebra in containing as a subalgebra, extends to an automorphism of .

We can thus try to characterize the -extensible automorphisms for various subvarieties of the variety of groups. Further, we do not need to restrict ourselves to varieties, and can instead look at the automorphisms extensible for particlar quasivarieties.

Also, there are analogous notions of pushforwardability and quotient-pullbackability for automorphisms and endomorphisms for any subvariety of the variety of groups.

Here are some particular problems:

- Fixed-class extensible endomorphisms problem: This problem asks for all the endomorphisms of a nilpotent group of class that can be extended to endomorphisms for all nilpotent groups of class containing it. The problem is interesting and nontrivial because there are endomorphisms of this kind that are neither trivial nor automorphisms.

Some cases that have been completely resolved:

### Extensible automorphisms problems involving order conditions on the group

- Hall-semidirectly extensible implies inner: A
**Hall-semidirectly extensible automorphism**is an automorphism that can be extended to any bigger group containing the given subgroup as a Hall subgroup with a normal complement (i.e., as a Hall retract). It turns out that any such automorphism is inner.

## Interpretations

### Destroying outer automorphisms

`Further information: Destroying outer automorphisms`

The extensible automorphisms problem, and its many variants, are based on the theme that the only automorphisms of a group that *survive* passing to bigger groups are the inner ones. In other words, outer automorphisms can be destroyed by passing to bigger groups.

A related result is the NPC theorem: it states that any normal subgroup can be realized as a characteristic subgroup inside some bigger group.

### Universal algebra and model theory

`Further information: Interpretation of the extensible automorphisms problem using universal algebra and model theory`

The extensible automorphisms problem, and specifically, the associated fact that extensible automorphisms are inner, can be interpreted as a statement about the nature of the variety of groups in terms of universal algebra, or of the theory of groups in terms of model theory/first-order logic. In these interpretations, we note that inner automorphisms are the only ones given by a *formula* that is guaranteed to hold for all groups.

### Use of representation-theoretic techniques

`Further information: Using group actions and representations to solve the extensible automorphisms problem, Conjugacy class-representation duality`

Both the use of linear representation theory to prove that finite-extensible automorphisms are class-preserving and the use of group actions to prove that extensible automorphisms are subgroup-conjugating share some common features. While the latter is mostly a straightforward application of the fundamental theorem of group actions that establishes a direct correspondence between subgroups and transitive group actions, the latter uses a more subtle conjugacy class-representation duality that allows one to relate linear representations with conjugacy classes.