Destroying outer automorphisms
This is a survey article related to:Extensible automorphisms problem
View other survey articles about Extensible automorphisms problem
This is a survey article related to:NPC conjecture
View other survey articles about NPC conjecture
This survey article is about a collection of related (and, for the most part, unsolved) problems on the theme: can outer automorphisms (and notions that depend on them) for a given group be destroyed by passing to bigger groups?
An inner automorphism of a group is an automorphism defined as conjugation by an element, and inner automorphisms of groups are remarkably averse to destruction. For instance, any inner automorphism of a subgroup can be extended to an inner automorphism of the whole group, while any inner automorphism of a quotient can be pulled back to an inner automorphism of the original group. This has to do with the fact that inner automorphisms have witnesses: the particular element used for conjugation, and as long as these witnesses are not destroyed, the automorphism is also intact. In the language of universal algebra, we say that inner automorphisms are I-automorphisms of the variety of groups. (In fact, they are the only ones).
Since outer automorphisms do not enjoy the protection given by witnesses, it may happen that they can be destroyed by passing to suitable supergroups or to groups having the original group as quotient. Even better, we may hope that the new bigger group has no outer automorphisms of its own. A related possibility is that normal subgroups of the original group may become characteristic subgroups in the bigger group.
We discuss conjectures related to each of these possibilities.
- 1 The extensible automorphisms problem and conjecture
- 2 Potentially characteristic subgroups
- 3 Single-witness ideas and extreme versions
The extensible automorphisms problem and conjecture
As the graphic illustrates, a little graffiti can completely destroy the symmetry of the circle. The graffiti idea can, in fact, be used to show that for a number of combinatorial structures, every object can be embedded in a bigger object with no nontrivial symmetries. For instance, every graph can be embedded as a subgraph of a bigger graph with no nontrivial symmetry.
With groups, the situation is different. If is a subgroup of , and is an inner automorphism of , can be extended to an inner automorphism of . In other words, there exists an inner automorphism of such that the restriction of to equals . The proof is essentially the fact that if is an element for which conjugation by gives , then conjugation by in gives .
A deeper algebraic explanation of this is that inner automorphisms are given by a formula, and this formula is guaranteed to give an automorphism. In fact, any automorphism arising from a formula guaranteed to give an automorphism must be inner. This general notion is called I-automorphism, and inner automorphisms are I-automorphisms in the variety of groups.
Extensible, pushforwardable, and quotient-pullbackable automorphisms
An automorphism of a group is termed an extensible automorphism if whenever is a group containing , there is an automorphism of such that the restriction of to equals .
An automorphism of a group is termed a pushforwardable automorphism if whenever is a homomorphism, there is an automorphism of such that . In other words, can be pushed forward across any homomorphism of groups. Note that pushforwardable automorphisms are extensible, since we can use subgroup inclusions as homomorphisms.
An automorphism of a group is termed a quotient-pullbackable automorphism if whenever is a surjective homomorphism, there is an automorphism of such that . In other words, can be pulled back across any homomorphism of groups.
Inner automorphisms are extensible (inner implies extensible), pushforwardable (inner implies pushforwardable) and quotient-pullbackable (inner implies quotient-pullbackable). The conjectures are about the converses of these statements holding:
- Extensible automorphisms conjecture: Every extensible automorphism of a group is inner.
- Pushforwardable automorphisms conjecture: Every pushforwardable automorphism of a group is inner.
- Quotient-pullbackable automorphisms conjecture: Every quotient-pullbackable automorphism of a group is inner.
Iterative variants of these notions
We can define the notion of -extensible for any ordinal . All automorphisms are -extensible, and an automorphism of a group is -extensible if, for any group containing as a subgroup, there exists an automorphism of that is -extensible, and such that the restriction of to is . For a limit ordinal, an automorphism is -extensible if it is -extensible for all ordinals .
An automorphism of a group is termed infinity-extensible if it is -extensible for every ordinal . We can, analogously, define -pushforwardable, -pushforwardable, -quotient-pullbackable, and -quotient-pullbackable.
Best known general results
The best known general results are that every extensible automorphism sends subgroups to conjugate subgroups, and every automorphism of a finite group that extends to an automorphism for any finite group containing it is inner. Many other results are known for variants of the problem that restrict to particular classes of groups or to particular kinds of embeddings.
Potentially characteristic subgroups
Definition and a conjecture
Since normality satisfies intermediate subgroup condition, any potentially characteristic subgroup is normal. Obviously, any characteristic subgroup is potentially characteristic, so the property of being potentially characteristic lies somewhere between characteristicity and normality.
The NPC conjecture states that every normal subgroup is potentially characteristic.
The NPC conjecture is closely related to the extensible automorphisms problem. While the extensible automorphisms problem says that outer automorphisms can be destroyed by taking bigger groups, the NPC conjecture says that invariance under all automorphisms (which is characteristicity) reduces to invariance under inner automorphisms once we quantify over all bigger groups. We discuss more of the relationship below.
The best known results related to the NPC conjecture include the finite NPC theorem, which states that the conjecture holds when we restrict all groups to finite groups. Also, there are results stating that finite normal subgroups, central subgroups, characteristic subgroups, as well as normal subgroups contained in a member of the upper central series are all potentially characteristic.
Potentially relatively characteristic subgroup
A subgroup of a group is termed a potentially relatively characteristic subgroup if there exists a group containing such that every automorphism of that restricts to an automorphism of also restricts to an automorphism of .
It turns out that being potentially relatively characteristic is equivalent to being normal. Note that this proof is specific to the variety of groups and the analogous statement does not hold when we restrict attention to subvarieties of the variety of groups.
Characteristic-/normal-potentially (relatively) characteristic
A subgroup of a group is termed a characteristic-potentially characteristic subgroup if there exists a group containing such that both and are characteristic in .
A subgroup of a group is termed a normal-potentially relatively characteristic subgroup if there exists a group containing such that both is normal in and is invariant under all automorphisms of that restrict to automorphisms of .
The implication chain is:
Characteristic-potentially characteristic Normal-potentially characteristic Normal-potentially relatively characteristic
(Note that characteristic-potentially relatively characteristic collapses to characteristic-potentially characteristic).
We might try to make the ambitious conjecture that every normal subgroup is normal-potentially relatively characteristic. This, however, is not true.
A normal-extensible automorphism is an automorphism that can be extended for any embedding as a normal subgroup of a bigger group. A subgroup of a group is termed a normal-extensible automorphism-invariant subgroup if it is invariant under all the normal-extensible automorphisms of the whole group.
It is not true that every normal-extensible automorphism is inner. In fact, a centerless group that is a maximal subgroup of its automorphism group has all its automorphisms normal-extensible, and in particular, it has normal-extensible outer automorphisms. More generally, every automorphism is center-fixing and inner automorphism group is maximal in automorphism group implies every automorphism is normal-extensible.
However, not every normal subgroup is invariant under normal-extensible automorphisms. This shows that not every normal subgroup is normal-potentially relatively characteristic. In particular, it shows that not every normal subgroup is normal-potentially characteristic, or characteristic-potentially characteristic.