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87 More about Stabilizer Chains
87.1 Generalized Conjugation Technique
87.2 The General Backtrack Algorithm with Ordered Partitions
87.2-1 Internal representation of ordered partitions
87.2-2 Functions for setting up an R-base
87.2-3 Refinement functions for the backtrack search
87.2-4 Functions for meeting ordered partitions
87.2-5 Avoiding multiplication of permutations
87.3 Stabilizer Chains for Automorphisms Acting on Enumerators
87.3-1 An operation domain for automorphisms
87.3-2 Enumerators for cosets of characteristic factors
87.3-3 Making automorphisms act on such enumerators

### 87 More about Stabilizer Chains

This chapter contains some rather technical complements to the material handled in the chapters 42 and 43.

#### 87.1 Generalized Conjugation Technique

The command `ConjugateGroup( G, p )` (see `ConjugateGroup` (39.2-6)) for a permutation group G with stabilizer chain equips its result also with a stabilizer chain, namely with the chain of G conjugate by p. Conjugating a stabilizer chain by a permutation p means replacing all the points which appear in the `orbit` components by their images under p and replacing every permutation g which appears in a `labels` or `transversal` component by its conjugate g^p. The conjugate g^p acts on the mapped points exactly as g did on the original points, i.e., (pnt.p). g^p = (pnt.g).p. Since the entries in the `translabels` components are integers pointing to positions of the `labels` list, the `translabels` lists just have to be permuted by p for the conjugated stabilizer. Then `generators` is reconstructed as `labels{ genlabels }` and `transversal{ orbit }` as `labels{ translabels{ orbit } }`.

This conjugation technique can be generalized. Instead of mapping points and permutations under the same permutation p, it is sometimes desirable (e.g., in the context of permutation group homomorphisms) to map the points with an arbitrary mapping map and the permutations with a homomorphism hom such that the compatibility of the actions is still valid: map(pnt).hom(g) = map(pnt.g). (Of course the ordinary conjugation is a special case of this, with map(pnt) = pnt.p and hom(g) = g^p.)

In the generalized case, the "conjugated" chain need not be a stabilizer chain for the image of hom, since the "preimage" of the stabilizer of map(b) (where b is a base point) need not fix b, but only fixes the preimage map^{-1}( map(b) ) setwise. Therefore the method can be applied only to one level and the next stabilizer must be computed explicitly. But if map is injective, we have map(b).hom(g) = map(b) if and only if b.g = b, and if this holds, then g = w(g_1, ..., g_n) is a word in the generators g_1, ..., g_n of the stabilizer of b and hom(g) =^* w( hom(g_1), ..., hom(g_n) ) is in the "conjugated" stabilizer. If, more generally, hom is a right inverse to a homomorphism φ (i.e., φ(hom(g)) = g for all g), equality * holds modulo the kernel of φ; in this case the "conjugated" chain can be made into a real stabilizer chain by extending each level with the generators of the kernel and appending a proper stabilizer chain of the kernel at the end. These special cases will occur in the algorithms for permutation group homomorphisms (see 40).

To "conjugate" the points (i.e., `orbit`) and permutations (i.e., `labels`) of the Schreier tree, a loop is set up over the `orbit` list constructed during the orbit algorithm, and for each vertex b with unique edge a(l)b ending at b, the label l is mapped with hom and b with map. We assume that the `orbit` list was built w.r.t. a certain ordering < of the labels, where l' < l means that every point in the orbit was mapped with l' before it was mapped with l. This shape of the `orbit` list is guaranteed if the Schreier tree is extended only by `AddGeneratorsExtendSchreierTree` (43.11-10), and it is then also guaranteed for the "conjugated" Schreier tree. (The ordering of the labels cannot be read from the Schreier tree, however.)

In the generalized case, it can happen that the edge a(l)b bears a label l whose image is "old", i.e., equal to the image of an earlier label l' < l. Because of the compatibility of the actions we then have map(b) = map(a).hom(l)^{-1} = map(a).hom(l')^{-1} = map(a{l'}^{-1}), so map(b) is already equal to the image of the vertex a{l'}^{-1}. This vertex must have been encountered before b = al^{-1} because l' < l. We conclude that the image of a label can be "old" only if the vertex at the end of the corresponding edge has an "old" image, too, but then it need not be "conjugated" at all. A similar remark applies to labels which map under hom to the identity.

#### 87.2 The General Backtrack Algorithm with Ordered Partitions

Section 43.12 describes the basic functions for a backtrack search. The purpose of this section is to document how the general backtrack algorithm is implemented in GAP and which parts you have to modify if you want to write your own backtrack routines.

##### 87.2-1 Internal representation of ordered partitions

GAP represents an ordered partition as a record with the following components.

`points`

  a list of all points contained in the partition, such that the points of each cell from lie consecutively,

`cellno`

  a list whose ith entry is the number of the cell which contains the point i,

`firsts`

  a list such that `points[firsts[j]]` is the first point in `points` which is in cell j,

`lengths`

  a list of the cell lengths.

Some of the information is redundant, e.g., the `lengths` could also be read off the `firsts` list, but since this need not be increasing, it would require some searching. Similar for `cellno`, which could be replaced by a systematic search of `points`, keeping track of what cell is currently being traversed. With the above components, the mth cell of a partition P is expressed as `P.points{ [ P.firsts[m] .. P.firsts[m] + P.lengths[m] - 1 ] }`. The most important operations, however, to be performed upon P are the splitting of a cell and the reuniting of the two parts. Following the strategy of J. Leon, this is done as follows:

(1)

  The points which make up the cell that is to be split are sorted so that the ones that remain inside occupy positions `[ P.firsts[m] .. last ]` in the list `P.points` (for a suitable value of last).

(2)

  The points at positions `[ last + 1 .. P.firsts[m] + P.lengths[m] - 1 ]` will form the additional cell. For this new cell requires additional entries are added to the lists `P.firsts` (namely, `last+1`) and `P.lengths` (namely, `P.firsts[m] + P.lengths[m] - last - 1`).

(3)

  The entries of the sublist `P.cellno{ [ last+1 .. P.firsts[m] + P.lengths[m]-1 ] }` must be set to the number of the new cell.

(4)

  The entry `P.lengths[m]` must be reduced to `last - P.firsts[m] + 1`.

Then reuniting the two cells requires only the reversal of steps 2 to 4 above. The list `P.points` need not be rearranged.

##### 87.2-2 Functions for setting up an R-base

This subsection explains some GAP functions which are local to the library file `lib/stbcbckt.gi` which contains the code for backtracking in permutation groups. They are mentioned here because you might find them helpful when you want to implement you own backtracking function based on the partition concept. An important argument to most of the functions is the R-base R, which you should regard as a black box. We will tell you how to set it up, how to maintain it and where to pass it as argument, but it is not necessary for you to know its internal representation. However, if you insist to learn the whole story: Here are the record components from which an R-base is made up:

`domain`

  the set Ω on which the group G operates

`base`

  the sequence (a_1, ..., a_r) of base points

`partition`

  an ordered partition, initially Π_0, this will be refined to Π_1, ..., Π_r during the backtrack algorithm

`where`

  a list such that a_i lies in cell number `where`[i] of Π_i

`rfm`

  a list whose ith entry is a list of refinements which take Σ_i to Σ_{i+1}; the structure of a refinement is described below

`chain`

  a (copy of a) stabilizer chain for G (not if G is a symmetric group)

`fix`

  only if G is a symmetric group: a list whose i entry contains `Fixcells(` Π_i`)`

`level`

  initially equal to `chain`, this will be changed to chains for the stabilizers G_{a_1 ... a_i} for i = 1, ..., r during the backtrack algorithm; if G is a symmetric group, only the number of moved points is stored for each stabilizer

`lev`

  a list whose ith entry remembers the `level` entry for G_{a_1 ... a_{i-1}}

`level2`, `lev2`

  a similar construction for a second group (used in intersection calculations), `false` otherwise. This second group H activated if the R-base is constructed as `EmptyRBase(` [ G, H ], Ω, Π_0`)` (if G = H, GAP sets `level2 =` `true` instead).

`nextLevel`

  this is described below

As our guiding example, we present code for the function `Centralizer` (35.4-4) which calculates the centralizer of an element g in the group G. (The real code is more general and has a few more subtleties.)

    Pi_0 := TrivialPartition( omega );
    R := EmptyRBase( G, omega, Pi_0 );
    R.nextLevel := function( Pi, rbase )
    local  fix, p, q, where;
    NextRBasePoint( Pi, rbase );
    fix := Fixcells( Pi );
    for p  in fix  do
      q := p ^ g;
      where := IsolatePoint( Pi, q );
      if where <> false  then
        Add( fix, q );
        ProcessFixpoint( R, q );
        AddRefinement( R, "Centralizer", [ Pi.cellno[ p ], q, where ] );
        if Pi.lengths[ where ] = 1  then
          p := FixpointCellNo( Pi, where );
          ProcessFixpoint( R, p );
          AddRefinement( R, "ProcessFixpoint", [ p, where ] );
        fi;
      fi;
    od;
    end;

    return PartitionBacktrack(
      G,
      c -> g ^ c = g,
      false,
      R,
      [ Pi_0, g ],
      L, R );

The list numbers below refer to the line numbers of the code above.

1.

  `omega` is the set on which `G` acts and `Pi_0` is the first member of the decreasing sequence of partitions mentioned in 43.12. We set `Pi_0 = omega`, which is constructed as `TrivialPartition( omega )`, but we could have started with a finer partition, e.g., into unions of `g`-cycles of the same length.

2.

  This statement sets up the R-base in the variable `R`.

3.-21.

  These lines define a function `R.nextLevel` which is called whenever an additional member in the sequence `Pi_0` ≥ Π_1 ≥ ... of partitions is needed. If Π_i does not yet contain enough base points in one-point cells, GAP will call `R.nextLevel(` Π_i,`R )`, and this function will choose a new base point a_{i+1}, refine Π_i to Π_{i+1} (thereby changing the first argument) and store all necessary information in `R`.

5.

  This statement selects a new base point a_{i+1}, which is not yet in a one-point cell of Π and still moved by the stabilizer G_{a_1 ... a_i} of the earlier base points. If certain points of `omega` should be preferred as base point (e.g., because they belong to long cycles of `g`), a list of points starting with the most wanted ones, can be given as an optional third argument to `NextRBasePoint` (actually, this is done in the real code for `Centralizer` (35.4-4)).

6.

  `Fixcells(` Π`)` returns the list of points in one-point cells of Π (ordered as the cells are ordered in Π).

7.

  For every point p ∈ fix, if we know the image p`^`g under c ∈ C_G(e), we also know ( p`^`g )`^`c = ( p`^`c )`^`g. We therefore want to isolate these extra points in Π.

9.

  This statement puts point q in a cell of its own, returning in `where` the number of the cell of Π from which q was taken. If q was already the only point in its cell, `where =` `false` instead.

12.

  This command does the necessary bookkeeping for the extra base point q: It prescribes q as next base in the stabilizer chain for G (needed, e.g., in line 5) and returns `false` if q was already fixed the stabilizer of the earlier base points (and `true` otherwise; this is not used here). Another call to `ProcessFixpoint` like this was implicitly made by the function `NextRBasePoint` to register the chosen base point. By contrast, the point q was not chosen this way, so `ProcessFixpoint` must be called explicitly for q.

13.

  This statement registers the function which will be used during the backtrack search to perform the corresponding refinements on the "image partition" Σ_i (to yield the refined Σ_{i+1}). After choosing an image b_{i+1} for the base point a_{i+1}, GAP will compute Σ_i ∧ ({ b_{i+1} }, Ω ∖ { b_{i+1} }) and store this partition in I`.partition`, where I is a black box similar to R, but corresponding to the current "image partition" (hence it is an "R-image" in analogy to the R-base). Then GAP will call the function `Refinements.Centralizer( R, I, Pi.cellno[ p ], p, where )`, with the then current values of R and I, but where Π`.cellno`[ p ], p, `where` still have the values they have at the time of this `AddRefinement` command. This function call will further refine I`.partition` to yield Σ_{i+1} as it is programmed in the function `Refinements.Centralizer`, which is described below. (The global variable `Refinements` is a record which contains all refinement functions for all backtracking procedures.)

14.-19.

  If the cell from which q was taken out had only two points, we now have an additional one-point cell. This condition is checked in line 13 and if it is true, this extra fixpoint p is taken (line 15), processed like q before (line 16) and is then (line 17) passed to another refinement function `Refinements.ProcessFixpoint( R, I, p, where )`, which is also described below.

23.-29.

  This command starts the backtrack search. Its result will be the centralizer as a subgroup of G. Its arguments are

24.

  the group we want to run through,

25.

  the property we want to test, as a GAP function,

26.

  `false` if we are looking for a subgroup, `true` in the case of a representative search (when the result would be one representative),

27.

  the R-base,

28.

  a list of data, to be stored in I`.data`, which has in position 1 the first member Σ_0 of the decreasing sequence of "image partitions" mentioned in 43.12. In the centralizer example, position 2 contains the element that is to be centralized. In the case of a representative search, i.e., a conjugacy test g`^`c`?=` h, we would have h instead of g here, and possibly a Σ_0 different from Π_0 (e.g., a partition into unions of h-cycles of same length).

29.

  two subgroups L ≤ C_G(g) and R ≤ C_G(h) known in advance (we have L = R in the centralizer case).

##### 87.2-3 Refinement functions for the backtrack search

The last subsection showed how the refinement process leading from Π_i to Π_{i+1} is coded in the function R`.nextLevel`, this has to be executed once the base point a_{i+1}. The analogous refinement step from Σ_i to Σ_{i+1} must be performed for each choice of an image b_{i+1} for a_{i+1}, and it will depend on the corresponding value of Σ_i ∧ ({b_{i+1}}, Ω ∖ {b_{i+1}}). But before we can continue our centralizer example, we must, for the interested reader, document the record components of the other black box I, as we did above for the R-base black box R. Most of the components change as GAP walks up and down the levels of the search tree.

`data`

  this will be mentioned below

`depth`

  the level i in the search tree of the current node Σ_i

`bimg`

  a list of images of the points in R`.base`

`partition`

  the partition Σ_i of the current node

`level`

  the stabilizer chain R`.lev`[i] at the current level

`perm`

  a permutation mapping `Fixcells`( Π_i ) to `Fixcells`( Σ_i ); this implies mapping (a_1, ..., a_i) to (b_1, ..., b_i)

`level2`, `perm2`

  a similar construction for the second stabilizer chain, `false` otherwise (and `true` if R`.level2 =` `true`)

As declared in the above code for `Centralizer` (35.4-4), the refinement is performed by the function `Refinement.Centralizer`( R, I, Π`.cellno`[p], p, where ). The functions in the record `Refinement` always take two additional arguments before the ones specified in the `AddRefinement` call (in line 13 above), namely the R-base R and the current value I of the "R-image". In our example, p is a fixpoint of Π = Π_i ∧ ({ a_{i+1} }, Ω ∖ { a_{i+1} }) such that where = Π`.cellno`[ p^g ]. The `Refinement` functions must return `false` if the refinement is unsuccessful (e.g., because it leads to Σ_{i+1} having different cell sizes from Π_{i+1}) and `true` otherwise. Our particular function looks like this.

    Refinements.Centralizer := function( R, I, cellno, p, where )
    local  Sigma, q;
    Sigma := I.partition;
    q := FixpointCellNo( Sigma, cellno ) ^ I.data[ 2 ];
    return IsolatePoint( Sigma, q ) = where and ProcessFixpoint( I, p, q );
    end;

The list numbers below refer to the line numbers of the code immediately above.

3.

  The current value of Σ_i ∧ ({ b_{i+1} }, Ω ∖ { b_{i+1} }) is always found in I`.partition`.

4.

  The image of the only point in cell number cellno = Π_i`.cellno`[ p ] in Σ under g = I`.data`[ 2 ] is calculated.

5.

  The function returns `true` only if the image q has the same cell number in Σ as p had in Π (i.e., where) and if q can be prescribed as an image for p under the coset of the stabilizer G_{a_1 ... a_{i+1}}.c where c ∈ G is an (already constructed) element mapping the earlier base points a_1, ..., a_{i+1} to the already chosen images b_1, ..., b_{i+1}. This latter condition is tested by `ProcessFixpoint`( I, p, q ) which, if successful, also does the necessary bookkeeping in I. In analogy to the remark about line 12 in the program above, the chosen image b_{i+1} for the base point a_{i+1} has already been processed implicitly by the function `PartitionBacktrack`, and this processing includes the construction of an element c ∈ G which maps `Fixcells`( Π_i ) to `Fixcells`( Σ_i ) and a_{i+1} to b_{i+1}. By contrast, the extra fixpoints p and q in Π_{i+1} and Σ_{i+1} were not chosen automatically, so they require an explicit call of `ProcessFixpoint`, which replaces the element c by some c'.c (with c' ∈ G_{a_1 ... a_{i+1}}) which in addition maps p to q, or returns `false` if this is impossible.

You should now be able to guess what `Refinements.ProcessFixpoint`( R, I, p, where ) does: it simply returns `ProcessFixpoint`( I, p,`FixpointCellNo`( I`.partition`, where ) ).

Summary.

When you write your own backtrack functions using the partition technique, you have to supply an R-base, including a component `nextLevel`, and the functions in the `Refinements` record which you need. Then you can start the backtrack by passing the R-base and the additional data (for the `data` component of the "R-image") to `PartitionBacktrack`.

##### 87.2-4 Functions for meeting ordered partitions

A kind of refinement that occurs in particular in the normalizer calculation involves computing the meet of Π (cf. lines 6ff. above) with an arbitrary other partition Λ, not just with one point. To do this efficiently, GAP uses the following two functions.

`StratMeetPartition(` R, Π, Λ `[`, g `] )`

`MeetPartitionStrat(` R, I`{,` Λ'`}[, {`g'`}]`, strat `)`

Such a `StratMeetPartition` command would typically appear in the function call R`.nextLevel`( Π, R ) (during the refinement of Π_i to Π_{i+1}). This command replaces Π by Π ∧ Λ (thereby changing the second argument) and returns a "meet strategy" strat. This is (for us) a black box which serves two purposes: First, it allows GAP to calculate faster the corresponding meet Σ ∧ Λ', which must then appear in a `Refinements` function (during the refinement of Σ_i to Σ_{i+1}). It is faster to compute Σ ∧ Λ' with the "meet strategy" of Π ∧ Λ because if the refinement of Σ is successful at all, the intersection of a cell from the left hand side of the ∧ sign with a cell from the right hand side must have the same size in both cases (and strat records these sizes, so that only non-empty intersections must be calculated for Σ ∧ Λ'). Second, if there is a discrepancy between the behaviour prescribed by strat and the behaviour observed when refining Σ, the refinement can immediately be abandoned.

On the other hand, if you only want to meet a partition Π with Λ for a one-time use, without recording a strategy, you can simply type `StratMeetPartition`( Π, Λ ) as in the following example, which also demonstrates some other partition-related commands.

    gap> P := Partition( [[1,2],[3,4,5],[6]] );;  Cells( P );
    [ [ 1, 2 ], [ 3, 4, 5 ], [ 6 ] ]
    gap> Q := Partition( OnTuplesTuples( last, (1,3,6) ) );;  Cells( Q );
    [ [ 3, 2 ], [ 6, 4, 5 ], [ 1 ] ]
    gap> StratMeetPartition( P, Q );
    [  ]
    gap> # The ``meet strategy'' was not recorded, ignore this result.
    gap> Cells( P );
    [ [ 1 ], [ 5, 4 ], [ 6 ], [ 2 ], [ 3 ] ]

You can even say `StratMeetPartition`( Π, ∆ ) where ∆ is simply a subset of Ω, it will then be interpreted as the partition (∆, Ω ∖ ∆).

GAP makes use of the advantages of a "meet strategy" if the refinement function in `Refinements` contains a `MeetPartitionStrat` command where strat is the "meet strategy" calculated by `StratMeetPartition` before. Such a command replaces I`.partition` by its meet with Λ', again changing the argument I. The necessary reversal of these changes when backtracking from a node (and prescribing the next possible image for a base point) is automatically done by the function `PartitionBacktrack`.

In all cases, an additional argument g means that the meet is to be taken not with Λ, but instead with Λ.{g^{-1}}, where operation on ordered partitions is meant cellwise (and setwise on each cell). (Analogously for the primed arguments.)

    gap> P := Partition( [[1,2],[3,4,5],[6]] );;
    gap> StratMeetPartition( P, P, (1,6,3) );;  Cells( P );
    [ [ 1 ], [ 5, 4 ], [ 6 ], [ 2 ], [ 3 ] ]

Note that P.(1,3,6) = Q.

##### 87.2-5 Avoiding multiplication of permutations

In the description of the last subsections, the backtrack algorithm constructs an element c ∈ G mapping the base points to the prescribed images and finally tests the property in question for that element. During the construction, c is obtained as a product of transversal elements from the stabilizer chain for G, and so multiplications of permutations are required for every c submitted to the test, even if the test fails (i.e., in our centralizer example, if g`^`c`<>`g). Even if the construction of c stops before images for all base points have been chosen, because a refinement was unsuccessful, several multiplications will already have been performed by (explicit or implicit) calls of `ProcessFixpoint`, and, actually, the general backtrack procedure implemented in GAP avoids this.

For this purpose, GAP does not actually multiply the permutations but rather stores all the factors of the product in a list. Specifically, instead of carrying out the multiplication in c ↦ c'.c mentioned in the comment to line 5 of the above program — where c' ∈ G_{a_1 ... a_{i+1}} is a product of factorized inverse transversal elements, see 43.9 — GAP appends the list of these factorized inverse transversal elements (giving c') to the list of factors already collected for c. Here c' is multiplied from the left and is itself a product of inverses of strong generators of G, but GAP simply spares itself all the work of inverting permutations and stores only a "list of inverses", whose product is then (c'.c)^{-1} (which is the new value of c^{-1}). The "list of inverses" is extended this way whenever `ProcessFixpoint` is called to improve c.

The product has to be multiplied out only when the property is finally tested for the element c. But it is often possible to delay the multiplication even further, namely until after the test, so that no multiplication is required in the case of an unsuccessful test. Then the test itself must be carried out with the factorized version of the element c. For this purpose, `PartitionBacktrack` can be passed its second argument (the property in question) in a different way, not as a single GAP function, but as a list like in lines 2–4 of the following alternative excerpt from the code for `Centralizer` (35.4-4).

    return PartitionBacktrack( G,
      [ g, g,
      OnPoints,
      c -> c!.lftObj = c!.rgtObj ],
      false, R, [ Pi_0, g ], L, R );

The test for c to have the property in question is of the form opr( left, c ) = right where opr is an operation function as explained in 41.12. In other words, c passes the test if and only if it maps a "left object" to a "right object" under a certain operation. In the centralizer example, we have opr`= OnPoints` and left = right = g, but in a conjugacy test, we would have right = h.

2.

  Two first two entries (here g and g) are the values of left and right.

3.

  The third entry (here `OnPoints` (41.2-1)) is the operation opr.

4.

  The fourth entry is the test to be performed upon the mapped left object left and preimage of the right object opr( right, c`^-1`). Here GAP operates with the inverse of c because this is the product of the permutations stored in the "list of inverses". The preimage of right under c is then calculated by mapping right with the factors of c^{-1} one by one, without the need to multiply these factors. This mapping of right is automatically done by the `ProcessFixpoint` function whenever c is extended, the current value of right is always stored in c`!.rgtObj`. When the test given by the fourth entry is finally performed, the element c has two components c`!.lftObj`= left and c`!.rgtObj`= opr( right, c`^-1`), which must be used to express the desired relation as a function of c. In our centralizer example, we simply have to test whether they are equal.

#### 87.3 Stabilizer Chains for Automorphisms Acting on Enumerators

This section describes a way of representing the automorphism group of a group as permutation group, following [ACM]. The code however is not yet included in the GAP library.

In this section we present an example in which objects we already know (namely, automorphisms of solvable groups) are equipped with the permutation-like operations `^` and `/` for action on positive integers. To achieve this, we must define a new type of objects which behave like permutations but are represented as automorphisms acting on an enumerator. Our goal is to generalize the Schreier-Sims algorithm for construction of a stabilizer chain to groups of such new automorphisms.

##### 87.3-1 An operation domain for automorphisms

The idea we describe here is due to C. Sims. We consider a group A of automorphisms of a group G, given by generators, and we would like to know its order. Of course we could follow the strategy of the Schreier-Sims algorithm (described in 43.6) for A acting on G. This would involve a call of `StabChainStrong( EmptyStabChain( [], One( A ) ), GroupGenerators( A ) )` where `StabChainStrong` is a function as the one described in the pseudo-code below:

    StabChainStrong := function( S, newgens )
      Extend the Schreier tree of S with newgens.
      for sch  in  Schreier generators  do
        if not sch in S.stabilizer  then
          StabChainStrong( S.stabilizer, [ sch ] );
        fi;
      od;
    end;

The membership test sch ∉ S`.stabilizer` can be performed because the stabilizer chain of S`.stabilizer` is already correct at that moment. We even know a base in advance, namely any generating set for G. Fix such a generating set (g_1, ..., g_d) and observe that this base is generally very short compared to the degree |G| of the operation. The problem with the Schreier-Sims algorithm, however, is then that the length of the first basic orbit g_1.A would already have the magnitude of |G|, and the basic orbits at deeper levels would not be much shorter. For the advantage of a short base we pay the high price of long basic orbits, since the product of the (few) basic orbit lengths must equal |A|. Such long orbits make the Schreier-Sims algorithm infeasible, so we have to look for a longer base with shorter basic orbits.

Assume that G is solvable and choose a characteristic series with elementary abelian factors. For the sake of simplicity we assume that N < G is an elementary abelian characteristic subgroup with elementary abelian factor group G/N. Since N is characteristic, A also acts as a group of automorphisms on the factor group G/N, but of course not necessarily faithfully. To retain a faithful action, we let A act on the disjoint union G/N with G, and choose as base (g_1 N, ..., g_d N, g_1, ..., g_d). Now the first d basic orbits lie inside G/N and can have length at most [G:N]. Since the base points g_1 N, ..., g_d N form a generating set for G/N, their iterated stabilizer A^(d+1) acts trivially on the factor group G/N, i.e., it leaves the cosets g_i N invariant. Accordingly, the next d basic orbits lie inside g_i N (for i = 1, ..., d) and can have length at most |N|.

Generalizing this method to a characteristic series G = N_0 > N_1 > ... > N_l = { 1 } of length l > 2, we can always find a base of length l.d such that each basic orbit is contained in a coset of a characteristic factor, i.e. in a set of the form g_i N_{j-1} / N_j (where g_i is one of the generators of G and 1 ≤ j ≤ l). In particular, the length of the basic orbits is bounded by the size of the corresponding characteristic factors. To implement a Schreier-Sims algorithm for such a base, we must be able to let automorphisms act on cosets of characteristic factors g_i N_{j-1} / N_j, for varying i and j. We would like to translate each such action into an action on { 1, ..., [ N_{j-1}:N_j] }, because then we need not enumerate the operation domain, which is the disjoint union of G / N_1, G / N_2 ... G / N_l, as a whole. Enumerating it as a whole would result in basic orbits like `orbit`⊆ { 1001, ..., 1100 } with a `transversal` list whose first 1000 entries would be unbound, but still require 4 bytes of memory each (see 43.9).

Identifying each coset g_i N_{j-1} / N_j into { 1, ..., [N_{j-1}:N_j] } of course means that we have to change the action of the automorphisms on every level of the stabilizer chain. Such flexibility is not possible with permutations because their effect on positive integers is "hardwired" into them, but we can install new operations for automorphisms.

##### 87.3-2 Enumerators for cosets of characteristic factors

So far we have not used the fact that the characteristic factors are elementary abelian, but we will do so from here on. Our first task is to implement an enumerator (see `AsList` (30.3-8) and 21.23) for a coset of a characteristic factor in a solvable group G. We assume that such a coset g N/M is given by

(1)

  a pcgs for the group G (see `Pcgs` (45.2-1)), let n =`Length(` pcgs`)`;

(2)

  a range range = [ start .. stop ] indicating that N = ⟨ pcgs{ [ start .. n ] } ⟩ and M = ⟨ pcgs{ [ stop + 1 .. n ] } ⟩, i.e., the cosets of pcgs{ range } form a base for the vector space N/M;

(3)

  the representative g.

We first define a new representation for such enumerators and then construct them by simply putting these three pieces of data into a record object. The enumerator should behave as a list of group elements (representing cosets modulo M), consequently, its family will be the family of the pcgs itself.

    DeclareRepresentation( "IsCosetSolvableFactorEnumeratorRep", IsEnumerator,
        [ "pcgs", "range", "representative" ] );

    EnumeratorCosetSolvableFactor := function( pcgs, range, g )
        return Objectify( NewType( FamilyObj( pcgs ),
                       IsCosetSolvableFactorEnumeratorRep ),
                       rec( pcgs := pcgs,
                           range := range,
                  representative := g ) );
    end;

The definition of the operations `Length` (21.17-5), `\[\]` (21.2-1) and `Position` (21.16-1) is now straightforward. The code has sometimes been abbreviated and is meant "cum grano salis", e.g., the declaration of the local variables has been left out.

    InstallMethod( Length, [ IsCosetSolvableFactorEnumeratorRep ],
        enum -> Product( RelativeOrdersPcgs( enum!.pcgs ){ enum!.range } ) );

    InstallMethod( \[\], [ IsCosetSolvableFactorEnumeratorRep,
            IsPosRat and IsInt ],
        function( enum, pos )
        elm := ();
        pos := pos - 1;
        for i  in Reversed( enum!.range )  do
            p := RelativeOrderOfPcElement( enum!.pcgs, i );
            elm := enum!.pcgs[ i ] ^ ( pos mod p ) * elm;
            pos := QuoInt( pos, p );
        od;
        return enum!.representative * elm;
    end );

    InstallMethod( Position, [ IsCosetSolvableFactorEnumeratorRep,
            IsObject, IsZeroCyc ],
        function( enum, elm, zero )
        exp := ExponentsOfPcElement( enum!.pcgs,
                       LeftQuotient( enum!.representative, elm ) );
        pos := 0;
        for i  in enum!.range  do
            pos := pos * RelativeOrderOfPcElement( pcgs, i ) + exp[ i ];
        od;
        return pos + 1;
    end );

##### 87.3-3 Making automorphisms act on such enumerators

Our next task is to make automorphisms of the solvable group pcgs`!.group` act on [ 1 ..`Length`( enum ) ] for such an enumerator enum. We achieve this by introducing a new representation of automorphisms on enumerators and by putting the enumerator together with the automorphism into an object which behaves like a permutation. Turning an ordinary automorphism into such a special automorphism requires then the construction of a new object which has the new type. We provide an operation `PermOnEnumerator( model, aut )` which constructs such a new object having the same type as model, but representing the automorphism aut. So aut can be either an ordinary automorphism or one which already has an enumerator in its type, but perhaps different from the one we want (i.e. from the one in model).

    DeclareCategory( "IsPermOnEnumerator",
        IsMultiplicativeElementWithInverse and IsPerm );

    DeclareRepresentation( "IsPermOnEnumeratorDefaultRep",
        IsPermOnEnumerator and IsAttributeStoringRep,
        [ "perm" ] );

    DeclareOperation( "PermOnEnumerator",
        [ IsEnumerator, IsObject ] );

    InstallMethod( PermOnEnumerator,
        [ IsEnumerator, IsObject ],
        function( enum, a )
        SetFilterObj( a, IsMultiplicativeElementWithInverse );
        a := Objectify( NewKind( PermutationsOnEnumeratorsFamily,
                     IsPermOnEnumeratorDefaultRep ),
                     rec( perm := a ) );
        SetEnumerator( a, enum );
        return a;
    end );

    InstallMethod( PermOnEnumerator,
        [ IsEnumerator, IsPermOnEnumeratorDefaultRep ],
        function( enum, a )
        a := Objectify( TypeObj( a ), rec( perm := a!.perm ) );
        SetEnumerator( a, enum );
        return a;
    end );

Next we have to install new methods for the operations which calculate the product of two automorphisms, because this product must again have the right type. We also have to write a function which uses the enumerators to apply such an automorphism to positive integers.

    InstallMethod( \*, IsIdenticalObj,
        [ IsPermOnEnumeratorDefaultRep, IsPermOnEnumeratorDefaultRep ],
        function( a, b )
        perm := a!.perm * b!.perm;
        SetIsBijective( perm, true );
        return PermOnEnumerator( Enumerator( a ), perm );
    end );

    InstallMethod( \^,
        [ IsPosRat and IsInt, IsPermOnEnumeratorDefaultRep ],
        function( p, a )
        return PositionCanonical( Enumerator( a ),
                       Enumerator( a )[ p ] ^ a!.perm );
    end );

How the corresponding methods for `p / aut` and `aut ^ n` look like is obvious.

Now we can formulate the recursive procedure `StabChainStrong` which extends the stabilizer chain by adding in new generators newgens. We content ourselves again with pseudo-code, emphasizing only the lines which set the `EnumeratorDomainPermutation`. We assume that initially S is a stabilizer chain for the trivial subgroup with a level for each pair (range,g) characterizing an enumerator (as described above). We also assume that the `identity` element at each level already has the type corresponding to that level.

    StabChainStrong := function( S, newgens )
      for i  in [ 1 .. Length( newgens ) ]  do
        newgens[ i ] := AutomorphismOnEnumerator( S.identity, newgens[ i ] );
      od;
      Extend the Schreier tree of S with newgens.
      for sch  in  Schreier generators  do
        if not sch in S.stabilizer  then
          StabChainStrong( S.stabilizer, [ sch ] );
        fi;
      od;
    end;

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