In
group theory
In abstract algebra, group theory studies the algebraic structures known as groups.
The concept of a group is central to abstract algebra: other well-known algebraic structures, such as rings, fields, and vector spaces, can all be seen as ...
, a branch of
mathematics, the
automorphism
In mathematics, an automorphism is an isomorphism from a mathematical object to itself. It is, in some sense, a symmetry of the object, and a way of mapping the object to itself while preserving all of its structure. The set of all automorphis ...
s and
outer automorphisms of the
symmetric groups and
alternating group
In mathematics, an alternating group is the group of even permutations of a finite set. The alternating group on a set of elements is called the alternating group of degree , or the alternating group on letters and denoted by or
Basic pr ...
s are both standard examples of these automorphisms, and objects of study in their own right, particularly the exceptional outer automorphism of S
6, the symmetric group on 6 elements.
Summary
Generic case
*
:
, and thus
.
:Formally,
is
complete
Complete may refer to:
Logic
* Completeness (logic)
* Completeness of a theory, the property of a theory that every formula in the theory's language or its negation is provable
Mathematics
* The completeness of the real numbers, which implies t ...
and the natural map
is an isomorphism.
*
:
, and the outer automorphism is conjugation by an
odd permutation
In mathematics, when ''X'' is a finite set with at least two elements, the permutations of ''X'' (i.e. the bijective functions from ''X'' to ''X'') fall into two classes of equal size: the even permutations and the odd permutations. If any total o ...
.
*
:
:Indeed, the natural maps
are isomorphisms.
Exceptional cases
*
: trivial:
::
::
*
:
*
:
, and
is a
semidirect product
In mathematics, specifically in group theory, the concept of a semidirect product is a generalization of a direct product. There are two closely related concepts of semidirect product:
* an ''inner'' semidirect product is a particular way in ...
.
*
:
, and
The exceptional outer automorphism of S6
Among symmetric groups, only S
6 has a non-trivial outer automorphism,
which one can call ''
exceptional'' (in analogy with
exceptional Lie algebras) or ''exotic''. In fact, Out(S
6) = C
2.
[ Lam, T. Y., & Leep, D. B. (1993). "Combinatorial structure on the automorphism group of S6". '' Expositiones Mathematicae'', 11(4), 289–308.]
This was discovered by
Otto Hölder in 1895.
The specific nature of the outer automorphism is as follows:
* the sole identity permutation maps to itself;
* a 2-cycle such as (1 2) maps to the product of three 2-cycles such as (1 2)(3 4)(5 6) and vice versa, there being 15 permutations each way;
* a 3-cycle such as (1 2 3) maps to the product of two 3-cycles such as (1 4 5)(2 6 3) and vice versa, accounting for 40 permutations each way;
* a 4-cycle such as (1 2 3 4) maps to another 4-cycle such as (1 6 2 4) accounting for 90 permutations;
* the product of two 2-cycles such as (1 2)(3 4) maps to another product of two 2-cycles such as (3 5)(4 6), accounting for 45 permutations;
* a 5-cycle such as (1 2 3 4 5) maps to other 5-cycles such as (1 3 6 5 2) accounting for 144 permutations;
* the product of a 2-cycle and a 3-cycle such as (1 2 3)(4 5) maps to a 6-cycle such as (1 2 5 3 4 6) and vice versa, accounting for 120 permutations each way;
* the product of a 2-cycle and a 4-cycle such as (1 2 3 4)(5 6) maps to another such permutation such as (1 4 2 6)(3 5) accounting for the 90 remaining permutations.
Thus, all 720 permutations on 6 elements are accounted for. The outer automorphism does not preserve cycle structure in general, mapping single cycles to the product of two cycles and vice versa.
This also yields another outer automorphism of A
6, and this is the only exceptional outer automorphism of a finite simple group: for the infinite families of simple groups, there are formulas for the number of outer automorphisms, and the simple group of order 360, thought of as A
6, would be expected to have two outer automorphisms, not four.
However, when A
6 is viewed as PSL(2, 9) the outer automorphism group has the expected order. (For
sporadic groups – i.e. those not falling in an infinite family – the notion of exceptional outer automorphism is ill-defined, as there is no general formula.)
Construction
There are numerous constructions, listed in .
Note that as an outer automorphism, it is a ''class'' of automorphisms, well-determined only up to an inner automorphism, hence there is not a natural one to write down.
One method is:
* Construct an exotic map (embedding) S
5 → S
6;
see below
* S
6 acts by conjugation on the six conjugates of this subgroup, yielding a map S
6 → S
''X'', where ''X'' is the set of conjugates. Identifying ''X'' with the numbers 1, ..., 6 (which depends on a choice of numbering of the conjugates, i.e., up to an element of S
6 (an inner automorphism)) yields an outer automorphism S
6 → S
6.
* This map is an outer automorphism, since a transposition does not map to a transposition, but inner automorphisms preserve cycle structure.
Throughout the following, one can work with the multiplication action on cosets or the conjugation action on conjugates.
To see that S
6 has an outer automorphism, recall that homomorphisms
from a group ''G'' to a symmetric group S
''n'' are essentially the same as actions
of ''G'' on a set of ''n'' elements, and the subgroup fixing a point is then a subgroup of
index
Index (or its plural form indices) may refer to:
Arts, entertainment, and media Fictional entities
* Index (''A Certain Magical Index''), a character in the light novel series ''A Certain Magical Index''
* The Index, an item on a Halo megastru ...
at most ''n'' in ''G''. Conversely if we have a subgroup of index ''n'' in ''G'', the action on the cosets gives a transitive action of ''G'' on ''n'' points, and therefore a homomorphism to S
''n''.
Construction from graph partitions
Before the more mathematically rigorous constructions, it helps to understand a simple construction.
Take a
complete graph
In the mathematical field of graph theory, a complete graph is a simple undirected graph in which every pair of distinct vertices is connected by a unique edge. A complete digraph is a directed graph in which every pair of distinct vertices is ...
with 6 vertices, K
6. It has 15 edges, which can be partitioned into
perfect matching
In graph theory, a perfect matching in a graph is a matching that covers every vertex of the graph. More formally, given a graph , a perfect matching in is a subset of edge set , such that every vertex in the vertex set is adjacent to exactl ...
s in 15 different ways, each perfect matching being a set of three edges no two of which share a vertex. It is possible to find a set of 5 perfect matchings from the set of 15 such that no two matchings share an edge, and that between them include all edges of the graph; this
graph factorization can be done in 6 different ways.
Consider a permutation of the 6 vertices, and see its effect on the 6 different factorizations. We get a map from 720 input permutations to 720 output permutations. That map is precisely the outer automorphism of S
6.
Being an automorphism, the map must preserve the order of elements, but it does not preserve cycle structure. For instance, a 2-cycle maps to a product of three 2-cycles; it is easy to see that a 2-cycle affects all of the 6 graph factorizations in some way, and hence has no fixed points when viewed as a permutation of factorizations. The fact that it is possible to construct this automorphism at all relies on a large number of numerical coincidences which apply only to .
Exotic map S5 → S6
There is a subgroup (indeed, 6 conjugate subgroups) of S
6 which is abstractly isomorphic to S
5, but which acts transitively as subgroups of S
6 on a set of 6 elements. (The image of the obvious map S
''n'' → S
''n''+1 fixes an element and thus is not transitive.)
Sylow 5-subgroups
Janusz and Rotman construct it thus:
* S
5 acts transitively by conjugation on the set of its 6
Sylow 5-subgroups, yielding an embedding S
5 → S
6 as a transitive subgroup of order 120.
This follows from inspection of 5-cycles: each 5-cycle generates a group of order 5 (thus a Sylow subgroup), there are 5!/5 = 120/5 = 24 5-cycles, yielding 6 subgroups (as each subgroup also includes the identity), and S
''n'' acts transitively by conjugation on the set of cycles of a given class, hence transitively by conjugation on these subgroups.
Alternately, one could use the Sylow theorems, which state generally that all Sylow p-subgroups are conjugate.
PGL(2,5)
The
projective linear group of dimension two over the
finite field with five elements, PGL(2, 5), acts on the
projective line over the field with five elements, P
1(F
5), which has six elements. Further, this action is
faithful and 3-
transitive, as is always the case for the action of the projective linear group on the projective line. This yields a map PGL(2, 5) → S
6 as a transitive subgroup. Identifying PGL(2, 5) with S
5 and the projective special linear group PSL(2, 5) with A
5 yields the desired exotic maps S
5 → S
6 and A
5 → A
6.
Following the same philosophy, one can realize the outer automorphism as the following two inequivalent actions of S
6 on a set with six elements:
* the usual action as a permutation group;
* the six inequivalent structures of an abstract 6-element set as the projective line P
1(F
5) – the line has 6 points, and the projective linear group acts 3-transitively, so fixing 3 of the points, there are 3! = 6 different ways to arrange the remaining 3 points, which yields the desired alternative action.
Frobenius group
Another way:
To construct an outer automorphism of S
6, we need to construct
an "unusual" subgroup of index 6 in S
6, in other words one that is not one of the six obvious S
5 subgroups fixing a point (which just correspond to inner automorphisms of S
6).
The
Frobenius group of
affine transformation
In Euclidean geometry, an affine transformation or affinity (from the Latin, ''affinis'', "connected with") is a geometric transformation that preserves lines and parallelism, but not necessarily Euclidean distances and angles.
More generally, ...
s of
F5 (maps ''x''
''ax'' + ''b'' where ''a'' ≠ 0) has order 20 = (5 − 1) · 5 and acts on the field with 5 elements, hence is a subgroup of S
5.
(Indeed, it is the normalizer of a Sylow 5-group mentioned above, thought of as the order-5 group of translations of F
5.)
S
5 acts transitively on the coset space, which is a set of 120/20 = 6 elements (or by conjugation, which yields the action above).
Other constructions
Ernst Witt found a copy of Aut(S
6) in the
Mathieu group
In group theory, a topic in abstract algebra, the Mathieu groups are the five sporadic simple groups ''M''11, ''M''12, ''M''22, ''M''23 and ''M''24 introduced by . They are multiply transitive permutation groups on 11, 12, 22, 23 or 24 obje ...
M
12 (a subgroup ''T'' isomorphic to S
6 and an element ''σ'' that normalizes ''T'' and acts by outer automorphism). Similarly to S
6 acting on a set of 6 elements in 2 different ways (having an outer automorphism), M
12 acts on a set of 12 elements in 2 different ways (has an outer automorphism), though since ''M''
12 is itself exceptional, one does not consider this outer automorphism to be exceptional itself.
The full automorphism group of A
6 appears naturally as a maximal subgroup of the Mathieu group M
12 in 2 ways, as either a subgroup fixing a division of the 12 points into a pair of 6-element sets, or as a subgroup fixing a subset of 2 points.
Another way to see that S
6 has a nontrivial outer automorphism is to use the fact that A
6 is isomorphic to PSL
2(9), whose automorphism group is the
projective semilinear group PΓL
2(9), in which PSL
2(9) is of index 4, yielding an outer automorphism group of order 4. The most visual way to see this automorphism is to give an interpretation via algebraic geometry over finite fields, as follows. Consider the action of S
6 on affine 6-space over the field k with 3 elements. This action preserves several things: the hyperplane ''H'' on which the coordinates sum to 0, the line ''L'' in ''H'' where all coordinates coincide, and the quadratic form ''q'' given by the sum of the squares of all 6 coordinates. The restriction of ''q'' to ''H'' has defect line ''L'', so there is an induced quadratic form ''Q'' on the 4-dimensional ''H''/''L'' that one checks is non-degenerate and non-split. The zero scheme of ''Q'' in ''H''/''L'' defines a smooth quadric surface ''X'' in the associated projective 3-space over ''k''. Over an algebraic closure of ''k'', ''X'' is a product of two projective lines, so by a descent argument ''X'' is the Weil restriction to ''k'' of the projective line over a quadratic étale algebra ''K''. Since ''Q'' is not split over ''k'', an auxiliary argument with special orthogonal groups over ''k'' forces ''K'' to be a field (rather than a product of two copies of ''k''). The natural S
6-action on everything in sight defines a map from S
6 to the ''k''-automorphism group of ''X'', which is the semi-direct product ''G'' of PGL
2(''K'') = PGL
2(9) against the Galois involution. This map carries the simple group A
6 nontrivially into (hence onto) the subgroup PSL
2(9) of index 4 in the semi-direct product ''G'', so S
6 is thereby identified as an index-2 subgroup of ''G'' (namely, the subgroup of ''G'' generated by PSL
2(9) and the Galois involution). Conjugation by any element of ''G'' outside of S
6 defines the nontrivial outer automorphism of S
6.
Structure of outer automorphism
On cycles, it exchanges permutations of type (12) with (12)(34)(56) (class 2
1 with class 2
3), and of type (123) with (145)(263) (class 3
1 with class 3
2). The outer automorphism also exchanges permutations of type (12)(345) with (123456) (class 2
13
1 with class 6
1). For each of the other cycle types in S
6, the outer automorphism fixes the class of permutations of the cycle type.
On A
6, it interchanges the 3-cycles (like (123)) with elements of class 3
2 (like (123)(456)).
No other outer automorphisms
To see that none of the other symmetric groups have outer automorphisms, it is easiest to proceed in two steps:
# First, show that any automorphism that preserves the
conjugacy class
In mathematics, especially group theory, two elements a and b of a group are conjugate if there is an element g in the group such that b = gag^. This is an equivalence relation whose equivalence classes are called conjugacy classes. In other wo ...
of transpositions is an inner automorphism. (This also shows that the outer automorphism of S
6 is unique; see below.) Note that an automorphism must send each conjugacy class (characterized by the
cyclic structure that its elements share) to a (possibly different) conjugacy class.
# Second, show that every automorphism (other than the above for S
6) stabilizes the class of transpositions.
The latter can be shown in two ways:
* For every symmetric group other than S
6, there is no other conjugacy class consisting of elements of order 2 that has the same number of elements as the class of transpositions.
* Or as follows:
Each permutation of order two (called an
involution) is a product of ''k'' > 0 disjoint transpositions, so that it has cyclic structure 2
''k''1
''n''−2''k''. What is special about the class of transpositions (''k'' = 1)?
If one forms the product of two distinct transpositions ''τ''
1 and ''τ''
2, then one always obtains either a 3-cycle or a permutation of type 2
21
''n''−4, so the order of the produced element is either 2 or 3. On the other hand, if one forms the product of two distinct involutions ''σ''
1, ''σ''
2 of type , then provided , it is always possible to produce an element of order 6, 7 or 4, as follows. We can arrange that the product contains either
* two 2-cycles and a 3-cycle (for ''k'' = 2 and ''n'' ≥ 7)
* a 7-cycle (for ''k'' = 3 and ''n'' ≥ 7)
* two 4-cycles (for ''k'' = 4 and ''n'' ≥ 8)
For ''k'' ≥ 5, adjoin to the permutations ''σ''
1, ''σ''
2 of the last example redundant 2-cycles that cancel each other, and we still get two 4-cycles.
Now we arrive at a contradiction, because if the class of transpositions is sent via the automorphism ''f'' to a class of involutions that has ''k'' > 1, then there exist two transpositions ''τ''
1, ''τ''
2 such that ''f''(''τ''
1) ''f''(''τ''
2) has order 6, 7 or 4, but we know that ''τ''
1''τ''
2 has order 2 or 3.
No other outer automorphisms of S6
S
6 has exactly one (class) of outer automorphisms: Out(S
6) = C
2.
To see this, observe that there are only two conjugacy classes of S
6 of size 15: the transpositions and those of class 2
3. Each element of Aut(S
6) either preserves each of these conjugacy classes, or exchanges them. Any representative of the outer automorphism constructed above exchanges the conjugacy classes, whereas an index 2 subgroup stabilizes the transpositions. But an automorphism that stabilizes the transpositions is inner, so the inner automorphisms form an index 2 subgroup of Aut(S
6), so Out(S
6) = C
2.
More pithily: an automorphism that stabilizes transpositions is inner, and there are only two conjugacy classes of order 15 (transpositions and triple transpositions), hence the outer automorphism group is at most order 2.
Small ''n''
Symmetric
For ''n'' = 2, S
2 = C
2 = Z/2 and the automorphism group is trivial (obviously, but more formally because Aut(Z/2) = GL(1, Z/2) = Z/2
* = C
1). The inner automorphism group is thus also trivial (also because S
2 is abelian).
Alternating
For ''n'' = 1 and 2, A
1 = A
2 = C
1 is trivial, so the automorphism group is also trivial. For ''n'' = 3, A
3 = C
3 = Z/3 is abelian (and cyclic): the automorphism group is GL(1, Z/3
*) = C
2, and the inner automorphism group is trivial (because it is abelian).
Notes
References
* https://web.archive.org/web/20071227060045/http://polyomino.f2s.com/david/haskell/outers6.html
Some Thoughts on the Number 6 by John Baez: relates outer automorphism to the
regular icosahedron
In geometry, a regular icosahedron ( or ) is a convex polyhedron with 20 faces, 30 edges and 12 vertices. It is one of the five Platonic solids, and the one with the most faces.
It has five equilateral triangular faces meeting at each vertex. It ...
* "12 points in PG(3, 5) with 95040 self-transformations" in "The Beauty of Geometry", by Coxeter: discusses outer automorphism on first 2 pages
*
*
*
* {{cite journal, jstor=2310241, title=On a Theorem of Hölder, first=Donald W., last=Miller, date=1 January 1958, journal=The American Mathematical Monthly, volume=65, issue=4, pages=252–254, doi=10.2307/2310241
Group theory
Finite groups
Permutation groups