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%W  examples.tex          FORMAT documentation  B. Eick and C.R.B. Wright
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\Chapter{Formation Examples}

%\index{Formation Examples}


The following is a {\GAP} session that illustrates the various functions
in the package.  We have chosen to work with the symmetric group $S_4$
and the special linear group $SL(2,3)$ as examples, because it is easy
to print and read the results of computations for these groups, and the
answers can be checked by inspection. However, both
$S_4$ and $SL(2,3)$ are extremely small examples for the algorithms in
\FORMAT. In
\cite{EW} we describe effective application of the algorithms  to groups
of  composition length as much as 61, for which the computations take
a few seconds to complete. The file `grp' contains some of these groups and other groups readable as GAP4 input.

\beginexample
gap> RequirePackage("format");;
\endexample
A primitive banner appears.

First we define $S_4$ as a permutation group and compute some 
subgroups of it.
\beginexample
gap> G := SymmetricGroup(4);
Sym( [ 1 .. 4 ] )
gap> SystemNormalizer(G);  CarterSubgroup(G);
Group([ (3,4) ])
Group([ (3,4), (1,4)(2,3), (1,3)(2,4) ])
\endexample
Now we take the formation of supersolvable groups from the examples
and look at it.
\beginexample
gap> sup := Formation("Supersolvable");
formation of Supersolvable groups 
gap> KnownAttributesOfObject(sup); KnownPropertiesOfObject(sup);
[ "NameOfFormation", "ScreenOfFormation" ]
[ "IsIntegrated" ]
\endexample 

We can look at the screen for `sup'.
\beginexample
gap> ScreenOfFormation(sup);
<Operation "AbelianExponentResidual">
gap> ScreenOfFormation(sup)(G,2); ScreenOfFormation(sup)(G,3);
Group([ (3,4), (2,4,3), (1,4)(2,3), (1,3)(2,4) ])
Group([ (2,4,3), (1,4)(2,3), (1,3)(2,4) ])
\endexample
We get the residuals for `G' of the formations of abelian groups of exponent 1 ($= 2-1$) and of exponent 2 (=$3-1$).

Notice that `sup' does not yet have a residual function.
 Let's compute some subgroups of `G' corresponding to `sup'.
\beginexample
gap> ResidualWrtFormation(G, sup);
Group([ (1,4)(2,3), (1,3)(2,4) ])
gap> KnownAttributesOfObject(sup);
[ "NameOfFormation", "ScreenOfFormation", "ResidualFunctionOfFormation" ]
\endexample
 The residual function for `sup' was required and created.
\beginexample
gap> FNormalizerWrtFormation(G, sup);
Group([ (3,4), (2,4,3) ])
gap> CoveringSubgroupWrtFormation(G, sup);
Group([ (3,4), (2,4,3) ])
gap> KnownAttributesOfObject(G);
[ "Size", "One", "SmallestMovedPoint", "NrMovedPoints", "MovedPoints", 
  "GeneratorsOfMagmaWithInverses", "TrivialSubmagmaWithOne", 
  "MultiplicativeNeutralElement", "DerivedSubgroup", "IsomorphismPcGroup", 
  "IsomorphismSpecialPcGroup", "Pcgs", "PcgsElementaryAbelianSeries", 
  "StabChainOptions", "ComputedResidualWrtFormations", 
  "ComputedAbelianExponentResiduals", "ComputedFNormalizerWrtFormations", 
  "ComputedCoveringSubgroup1s", "ComputedCoveringSubgroup2s", 
  "SystemNormalizer", "CarterSubgroup" ]
\endexample
 The `AbelianExponentResidual's were computed in connection with the
local definition of `sup'. (`AbelianExponentResidual(G, n)' returns
the smallest normal subgroup of `G' whose factor group is abelian of
exponent dividing `n-1'.) Here are some of the other records.
\beginexample
gap> ComputedResidualWrtFormations(G);
[ formation of Supersolvable groups , Group([ (1,4)(2,3), (1,3)(2,4) ]) ]
gap> ComputedFNormalizerWrtFormations(G);
[ formation of Nilpotent groups , Group([ (3,4) ]), 
  formation of Supersolvable groups , Group([ (3,4), (2,4,3) ]) ]
gap> ComputedCoveringSubgroup2s(G);
[  ]
gap> ComputedCoveringSubgroup1s(G);
[ formation of Nilpotent groups , Group([ (3,4), (1,3)(2,4), (1,2)(3,4) ]), 
  formation of Supersolvable groups , Group([ (3,4), (2,4,3) ]) ]
\endexample
The call by `CoveringSubgroupWrtFormation' was to `CoveringSubgroup1', not
`CoveringSubgroup2'.

We could also have started with a pc group or a nice enough matrix group.
\beginexample
gap> s4 := SmallGroup(IdGroup(G));
<pc group of size 24 with 4 generators>
\endexample
This is $S_4$ again. The answers just look different now.
\beginexample
gap> SystemNormalizer(s4); CarterSubgroup(s4);
Group([ f1 ])
Group([ f1, f4, f3*f4 ])
\endexample
Similarly, we have $SL(2,3)$ and an isomorphic pc group.
\beginexample
gap> sl := SpecialLinearGroup(2,3);
SL(2,3)
gap> h := SmallGroup(IdGroup(sl));
<pc group of size 24 with 4 generators>
\endexample
We get the following subgroups.
\beginexample
gap> CarterSubgroup(sl); Size(last);
<group of 2x2 matrices in characteristic 3>
6
gap> SystemNormalizer(h); CarterSubgroup(h);
Group([ f1, f4 ])
Group([ f1, f4 ])
\endexample

Now let's make new formations from old.
\beginexample
gap> ab := Formation("Abelian");
formation of Abelian groups 
gap> KnownPropertiesOfObject(ab); KnownAttributesOfObject(ab);
[  ]
[ "NameOfFormation", "ResidualFunctionOfFormation" ]
gap> nil2 := Formation("PNilpotent",2);
formation of 2Nilpotent groups 
gap> KnownPropertiesOfObject(nil2); KnownAttributesOfObject(nil2);
[ "IsIntegrated" ]
[ "NameOfFormation", "ScreenOfFormation", "ResidualFunctionOfFormation" ]
\endexample
Compute the product and check some attributes.
\beginexample
gap> form := ProductOfFormations(ab, nil2);
formation of (AbelianBy2Nilpotent) groups 
gap> KnownAttributesOfObject(form);
[ "NameOfFormation", "ResidualFunctionOfFormation" ]
\endexample
Now the product in the other order, which *is* locally defined.
\beginexample
gap> form2 := ProductOfFormations(nil2, ab);
formation of (2NilpotentByAbelian) groups 
gap> KnownAttributesOfObject(form2);
[ "NameOfFormation", "ScreenOfFormation", "ResidualFunctionOfFormation" ]
\endexample
We check the results on `G', which is still $S_4$.
\beginexample
gap> ResidualWrtFormation(G, form);  ResidualWrtFormation(G, form2);
Group(())
Group([ (1,3)(2,4), (1,4)(2,3) ])
gap> KnownPropertiesOfObject(form2);
[  ]
\endexample
Although `form2' is not integrated, we can make an integrated formation
that differs from `form2' only in its local definition, i.e., whose
residual subgroups are the same as those for `form2'.
\beginexample
gap> Integrated(form2);
formation of (2NilpotentByAbelian)Int groups 
\endexample
`FNormalizerWrtFormation' and
`CoveringSubgroupWrtFormation' both require integrated formations, so they
silently replace `form2' by this last formation without, however,
changing `form2'. 
\beginexample
gap> FNormalizerWrtFormation(G, form2); CoveringSubgroupWrtFormation(G, form2);
Group([ (3,4), (2,4,3) ])
Group([ (3,4), (2,4,3) ])
gap> KnownPropertiesOfObject(form2);
[  ]
gap> ComputedCoveringSubgroup1s(G);
[ formation of (2NilpotentByAbelian)Int groups , Group([ (3,4), (2,4,3) ]), 
  formation of Nilpotent groups , Group([ (3,4), (1,4)(2,3), (1,3)(2,4) ]), 
  formation of Supersolvable groups , Group([ (3,4), (2,4,3) ]) ]
gap> ComputedResidualWrtFormations(G);
[ formation of (2NilpotentByAbelian) groups , 
  Group([ (1,3)(2,4), (1,2)(3,4) ]), 
  formation of (AbelianBy2Nilpotent) groups , Group(()), 
  formation of 2Nilpotent groups , Group([ (1,2)(3,4), (1,3)(2,4) ]), 
  formation of Abelian groups , Group([ (1,3,2), (2,4,3) ]), 
  formation of Supersolvable groups , Group([ (1,4)(2,3), (1,3)(2,4) ]) ]
\endexample
Lots of work has been going on behind the scenes.

Before we compute an intersection, we construct yet another formation.
\beginexample
gap> pig := Formation("PiGroups", [2,5]);
formation of (2,5)-Group groups with support [ 2, 5 ]
gap> form := Intersection(pig, nil2);
formation of ((2,5)-GroupAnd2Nilpotent) groups with support [ 2, 5 ]
gap> KnownAttributesOfObject(form);
[ "NameOfFormation", "ScreenOfFormation", "SupportOfFormation", 
  "ResidualFunctionOfFormation" ]
\endexample
 Let's cut down the support of `nil2' to $\{2,5\}$.
\beginexample
gap> form3 := ChangedSupport(nil2, [2,5]);
formation of Changed2Nilpotent[ 2, 5 ] groups 
gap> SupportOfFormation(form3);
[ 2, 5 ]
gap> form = form3;
false
\endexample
Although the formations defined by `form' and `form3' are abstractly
identical, {\GAP} has no way to know this fact, and so distinguishes
them.

We can mix the various operations, too.
\beginexample
gap> ProductOfFormations(Intersection(pig, nil2), sup);
formation of (((2,5)-GroupAnd2Nilpotent)BySupersolvable) groups 
gap> Intersection(pig, ProductOfFormations(nil2, sup));
formation of ((2,5)-GroupAnd(2NilpotentBySupersolvable)) groups with support 
[ 2, 5 ]
\endexample

  Now let's define our own formation.
\beginexample
gap> preform := rec( name := "MyOwn", 
>  fScreen := function( G, p)
>  return DerivedSubgroup( G );
>  end);
rec( name := "MyOwn", fScreen := function( G, p ) ... end )
gap> form := Formation(preform);
formation of MyOwn groups 
gap> KnownAttributesOfObject(form); KnownPropertiesOfObject(form);
[ "NameOfFormation", "ScreenOfFormation" ]
[  ]
\endexample
In fact, the definition is integrated. Let's tell {\GAP} so and compute
some related subgroups.
\beginexample
gap> SetIsIntegrated(form, true);
gap> ResidualWrtFormation(G, form);
Group([ (1,3)(2,4), (1,2)(3,4) ])
gap> FNormalizerWrtFormation(G, form);
Group([ (3,4), (2,4,3) ])
gap> CoveringSubgroup1(G, form);
Group([ (3,4), (2,4,3) ])
\endexample
These answers are consistent with the fact that MyOwn is really just the
formation of abelian by nilpotent groups.
