notes-RT--Groups-and-GAP.md

---

title   : Groups and GAP
date    : 2021-07-06
tags    : math, GAP, computer algebra
summary : A quick overview of some tools for carrying out "computer algebra" calculations.

---

Note: In class, I promised I'd include these in the notes... you'll have to read the docs a bit to figure out how to get Character Tables. (The notes her might help you get started).

A overview of GAP

GAP is

GAP - Groups, Algorithms, Programming - a System for Computational Discrete Algebra

You can find lots of information at the preceding link, including a tutorial and a variety of learning resources.

My basic goal here is to try to give you some idea of what you might be able to do with GAP (something of a trailer for the tutorial, perhaps?)

Installation

The installation instructions for GAP can be found here.

Personally, my computer's OS is Debian GNU/Linux, and I installed GAP by running the command

$ apt install gap

But while that is handy, I don't think the install process should be too difficult on any machine.

Starting GAP

After installing GAP, open a terminal on the computer and run GAP; you should see something like the following:

$ gap
 ┌───────┐   GAP 4.11.0 of 29-Feb-2020
 │  GAP  │   https://www.gap-system.org
 └───────┘   Architecture: x86_64-pc-linux-gnu-default64-kv7
 Configuration:  gmp 6.2.1, GASMAN, readline
 Loading the library and packages ...
 Packages:   Alnuth 3.1.2, AtlasRep 2.1.0, AutPGrp 1.10.2, CTblLib 1.3.1, 
             FactInt 1.6.3, GAPDoc 1.6.3, IO 4.7.0, Polycyclic 2.15.1, 
             PrimGrp 3.4.0, SmallGrp 1.4.1, TomLib 1.2.9, TransGrp 2.0.6
 Try '??help' for help. See also '?copyright', '?cite' and '?authors'
gap> 

The bit at the end -- i.e. gap> -- is a prompt, and it indicates that GAP is waiting for you to type a command. Once you do so and press Enter, GAP will (try to) execute the command you entered.

It will print any output (or errors) from the command on the screen, and then it will display another prompt and again wait for input.

Permutation Groups

In GAP, probably the most straightforward way of representing (finite) groups is as permutation groups -- i.e. subgroups of $S_n$ for some $n \ge 1$.

For example, one knows that the symmetric group $S_n$ is generated by an $n$-cycle and a transposition. In GAP, we may check this for some chosen $n$ -- say, $n=8$ -- as follows:

gap> G := Group((1,2),(1,2,3,4,5,6,7,8));
Group([ (1,2), (1,2,3,4,5,6,7,8) ])
gap> Order(G) = Factorial(8);
true

Note that elements are represented as products of disjoint cycles. If you enter them otherwise, GAP rewrites your representation as a product of disjoint cycles:

gap> (1,2,3,4)*(1,2,5);
(1,5)(2,3,4)

We can ask about the derived subgroup of $G$:

gap> H := DerivedSubgroup(G);
Group([ (1,2,3), (2,4,3), (2,4,5), (2,5,6,3,4), (3,7,4), (2,6)(4,7,8,5) ])
gap> Order(H) = Factorial(8)/2;
true

And we can ask about simplicity of groups:

gap> IsSimple(H);
true
gap> IsSimple(G);
false

We can ask about Sylow subgroups:

gap> f:=Factors(Order(G));
[ 2, 2, 2, 2, 2, 2, 2, 3, 3, 5, 7 ]
gap> Set(f);
[ 2, 3, 5, 7 ]
gap> ps:=List(Set(f),p->[p,SylowSubgroup(G,p)]);
[ [ 2, Group([ (1,2), (3,4), (1,3)(2,4), (5,6), (7,8), (5,7)(6,8), (1,5)(2,6)
          (3,7)(4,8) ]) ], [ 3, Group([ (1,2,3), (4,5,6) ]) ], 
  [ 5, Group([ (1,2,3,4,5) ]) ], [ 7, Group([ (1,2,3,4,5,6,7) ]) ] ]
gap> for x in ps do
  Print("p=",x[1]," Sylow order=",Order(x[2]),
        " Sylow abelian? ",IsAbelian(x[2]),"\n");
od;
p=2 Sylow order=128 Sylow abelian? false
p=3 Sylow order=9 Sylow abelian? true
p=5 Sylow order=5 Sylow abelian? true
p=7 Sylow order=7 Sylow abelian? true

Let's define a function that takes a (finite) group as an argument and prints the "sylow information".

SylowInfo:=function(G)
  ps:=List(Set(Factors(Order(G))),p->[p,SylowSubgroup(G,p)]);
  for x in ps do
    Print("p=",x[1]," Sylow order=",Order(x[2]),
        " Sylow abelian? ",IsAbelian(x[2]),"\n");
  od;
  return ps;
end

If we enter the above function in GAP, we can call it for different groups:

gap> SylowInfo(G);
p=2 Sylow order=128 Sylow abelian? false
p=3 Sylow order=9 Sylow abelian? true
p=5 Sylow order=5 Sylow abelian? true
p=7 Sylow order=7 Sylow abelian? true
[ [ 2, Group([ (1,2), (3,4), (1,3)(2,4), (5,6), (7,8), (5,7)(6,8), (1,5)(2,6)
          (3,7)(4,8) ]) ], [ 3, Group([ (1,2,3), (4,5,6) ]) ], 
  [ 5, Group([ (1,2,3,4,5) ]) ], [ 7, Group([ (1,2,3,4,5,6,7) ]) ] ]


gap> SylowInfo(H);
p=2 Sylow order=64 Sylow abelian? false
p=3 Sylow order=9 Sylow abelian? true
p=5 Sylow order=5 Sylow abelian? true
p=7 Sylow order=7 Sylow abelian? true
[ [ 2, Group([ (1,2)(3,4), (1,3)(2,4), (1,2)(5,6), (1,2)(7,8), (5,7)(6,8), 
          (1,5)(2,6)(3,7)(4,8) ]) ], [ 3, Group([ (1,2,3), (4,5,6) ]) ], 
  [ 5, Group([ (1,2,3,4,5) ]) ], [ 7, Group([ (1,2,3,4,5,6,7) ]) ] ]

gap> SylowInfo(SymmetricGroup(13));
p=2 Sylow order=1024 Sylow abelian? false
p=3 Sylow order=243 Sylow abelian? false
p=5 Sylow order=25 Sylow abelian? true
p=7 Sylow order=7 Sylow abelian? true
p=11 Sylow order=11 Sylow abelian? true
p=13 Sylow order=13 Sylow abelian? true
[ [ 2, <permutation group of size 1024 with 10 generators> ], 
  [ 3, Group([ (1,2,3), (4,5,6), (7,8,9), (1,4,7)(2,5,8)(3,6,9), (10,11,12) 
         ]) ], [ 5, Group([ (1,2,3,4,5), (6,7,8,9,10) ]) ], 
  [ 7, Group([ (1,2,3,4,5,6,7) ]) ], 
  [ 11, Group([ (1,2,3,4,5,6,7,8,9,10,11) ]) ], 
  [ 13, Group([ (1,2,3,4,5,6,7,8,9,10,11,12,13) ]) ] ]
  
gap> SylowInfo(AlternatingGroup(14));
p=2 Sylow order=1024 Sylow abelian? false
p=3 Sylow order=243 Sylow abelian? false
p=5 Sylow order=25 Sylow abelian? true
p=7 Sylow order=49 Sylow abelian? true
p=11 Sylow order=11 Sylow abelian? true
p=13 Sylow order=13 Sylow abelian? true
[ [ 2, <permutation group of size 1024 with 10 generators> ], 
  [ 3, Group([ (1,2,3), (4,5,6), (7,8,9), (1,4,7)(2,5,8)(3,6,9), (10,11,12) 
         ]) ], [ 5, Group([ (1,2,3,4,5), (6,7,8,9,10) ]) ], 
  [ 7, Group([ (1,2,3,4,5,6,7), (8,9,10,11,12,13,14) ]) ], 
  [ 11, Group([ (1,2,3,4,5,6,7,8,9,10,11) ]) ], 
  [ 13, Group([ (1,2,3,4,5,6,7,8,9,10,11,12,13) ]) ] ]  

Finally, I'll point out that we can ask about group isomorphisms.

Let's look at a dihedral group:

gap> sigma:=function(n) 
  return CycleFromList([1..n])*CycleFromList([n+1..2*n]);
end;
function( n ) ... end

gap> sigma(9);
(1,2,3,4,5,6,7,8,9)(10,11,12,13,14,15,16,17,18)
  # element of S_18 of order 9

gap> tau:= function(n)
  return Product(List([1..n],i->(i,2*n-i+1)));
function( n ) ... end

gap> tau(9);
(1,18)(2,17)(3,16)(4,15)(5,14)(6,13)(7,12)(8,11)(9,10)
  # element of S_18 of order 2

Note that "tau * sigma * tau = sigma^-1". e.g.

gap> tau(9)*sigma(9)*tau(9)=sigma(9)^-1;
true

gap> MyDihedralGroup:=function(n)
  return Group(sigma(n),tau(n));
end;
function( n ) ... end

We've now defined the function MyDihedralGroup, and MyDihedralGroup(n) returns the subgroup of $S_{2n}$ generated by sigma(n) and tau(n).

Now, GAP has a built-in function DihedralGroup; here DihedralGroup(n) returns a copy of the dihedral group of order n (so n needs to be even...).

Let's check that we get isomorphic groups:

gap> DihedralGroup(18);
<pc group of size 18 with 3 generators>

gap> MyDihedralGroup(9);
Group([ (1,2,3,4,5,6,7,8,9)(10,11,12,13,14,15,16,17,18), (1,18)(2,17)(3,16)
  (4,15)(5,14)(6,13)(7,12)(8,11)(9,10) ])
  
gap> IsomorphismGroups(DihedralGroup(18),MyDihedralGroup(9));
[ f1, f2, f3 ] -> [ (1,18)(2,17)(3,16)(4,15)(5,14)(6,13)(7,12)(8,11)(9,10), 
  (1,2,3,4,5,6,7,8,9)(10,11,12,13,14,15,16,17,18), 
  (1,4,7)(2,5,8)(3,6,9)(10,13,16)(11,14,17)(12,15,18) ]

The function IsomorphismGroups takes two groups as arguments, and returns an isomorphism (if one exists) or "fails". In the preceding case, GAP found an explicit isomorphism between GAP's DihedralGroup(18) and MyDihedralGroup(9).

You can check it for other values of n...

gap> IsomorphismGroups(DihedralGroup(32),MyDihedralGroup(16));
[ f1, f2, f3, f4, f5 ] -> 
[ (1,32)(2,31)(3,30)(4,29)(5,28)(6,27)(7,26)(8,25)(9,24)(10,23)(11,22)(12,
    21)(13,20)(14,19)(15,18)(16,17), (1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,
    16)(17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32), 
  (1,3,5,7,9,11,13,15)(2,4,6,8,10,12,14,16)(17,19,21,23,25,27,29,31)(18,20,22,
    24,26,28,30,32), (1,5,9,13)(2,6,10,14)(3,7,11,15)(4,8,12,16)(17,21,25,
    29)(18,22,26,30)(19,23,27,31)(20,24,28,32), 
  (1,9)(2,10)(3,11)(4,12)(5,13)(6,14)(7,15)(8,16)(17,25)(18,26)(19,27)(20,
    28)(21,29)(22,30)(23,31)(24,32) ]

And in the other order:

gap> IsomorphismGroups(MyDihedralGroup(16),DihedralGroup(32));	
[ (1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16)(17,18,19,20,21,22,23,24,25,26,27,
    28,29,30,31,32), (1,32)(2,31)(3,30)(4,29)(5,28)(6,27)(7,26)(8,25)(9,
    24)(10,23)(11,22)(12,21)(13,20)(14,19)(15,18)(16,17) ] -> [ f2, f1 ]

And you can observe that it fails on nonsense:

gap> IsomorphismGroups(DihedralGroup(30),MyDihedralGroup(16));
fail

Matrix groups over finite fields

Finite fields

Recall the following:

• a finite field $\mathbb{F} = \mathbb{F}_q$ has order $q=p^n$ for some prime $p$ and some $n \ge 1$

• the multiplicative group $\mathbb{F}_q^\times$ is cyclic of order $q-1$.

GAP writes Z(p,n) or Z(q) for a generator of the multiplicative group $\mathbb{F}_q^\times$.

Now you can perform field operations with these elements:

gap> Z(9) + 5*Z(3) + 2*Z(9)*Z(3);
Z(3^2)^3

gap> Z(9) + Z(27);  # note F_9 is not contained in F_27;
                    # the smallest field containing both F_9 and  
                    # F_27 is F_3^6=F_729
Z(3^6)^297

You can get the elements 0 or 1 viewed as members of a finite field using 0*Z(p,n) for 0 and Z(p,n)^0 for 1.

Matrices

A row vector is just a list; e.g.

gap> [ 1, 2, 3, 4 ];
[ 1, 2, 3, 4 ]

A matrix is a list of row vectors:

gap> [ [ 1, 1 ], [ 0, 1 ] ];   # integer entries...
[ [ 1, 1 ], [ 0, 1 ] ]

gap> [ [ 1, 1 ], [ 0, 1 ] ]*Z(9)^0;   # entries in the field F_9.
[ [ Z(3)^0, Z(3)^0 ], [ 0*Z(3), Z(3)^0 ] ]

Matrix Groups

In GAP, we can create the general linear group $\operatorname{GL}_n(\mathbb{F}_q)$ of invertible $n\times n$ matrices over the field of order $q$ using a command of the form G:=GL(n,q);; for example:

gap> G:=GL(3,17);
GL(3,17)
gap> Order(G);
111203278848

Similarly, we can construct the special linear group -- the kernel of

$$\det:\operatorname{GL}_n(\mathbb{F}_q) \to \mathbb{F}_q^\times$$

-- using SL(n,q).

GAP has constructors for other matrix groups (projective special linear group, symplectic group, unitary group , orthogonal group, ...) which I'm going to ignore today.

It is a fact that unless (n,q) ∈ [(2,2),(2,3)] one knows that SL(n,q) is a simple group $\iff \gcd(n,q-1) =1$, which we can confirm in some cases in GAP:

gap> IsSimple(SL(5,7));
true
gap> IsSimple(SL(3,7));
false

In fact, the quotient of SL(n,q) by its center is simple (at least when $n$ and $q$ aren't "tiny"):

gap> ZZ:=Center(SL(3,7)); Order(ZZ);
<group of 3x3 matrices over GF(7)>
3
gap> P:=SL(3,7)/ZZ; Order(P); IsSimple(P);
<permutation group with 2 generators>
1876896
true

GAP can compute the conjugacy classes of a finite group:

gap> ConjugacyClasses(SL(2,7));
[ [ [ Z(7)^0, 0*Z(7) ], [ 0*Z(7), Z(7)^0 ] ]^G, 
  [ [ 0*Z(7), Z(7)^3 ], [ Z(7)^0, Z(7)^5 ] ]^G, 
  [ [ 0*Z(7), Z(7)^4 ], [ Z(7)^5, Z(7)^5 ] ]^G, 
  [ [ Z(7)^3, 0*Z(7) ], [ 0*Z(7), Z(7)^3 ] ]^G, 
  [ [ 0*Z(7), Z(7)^3 ], [ Z(7)^0, Z(7)^2 ] ]^G, 
  [ [ 0*Z(7), Z(7)^4 ], [ Z(7)^5, Z(7)^2 ] ]^G, 
  [ [ 0*Z(7), Z(7)^3 ], [ Z(7)^0, 0*Z(7) ] ]^G, 
  [ [ 0*Z(7), Z(7)^3 ], [ Z(7)^0, Z(7)^4 ] ]^G, 
  [ [ 0*Z(7), Z(7)^3 ], [ Z(7)^0, Z(7) ] ]^G, 
  [ [ Z(7)^4, 0*Z(7) ], [ 0*Z(7), Z(7)^2 ] ]^G, 
  [ [ Z(7)^5, 0*Z(7) ], [ 0*Z(7), Z(7) ] ]^G ]

For each of these conjugacy classes, one may use the function Representative to choose a representative element.

Let's write some code that takes as input a whole number n, a prime power q=p^m and a matrix M and finds the conjugacy class in GL(n,q) containing M*Z(q).

LocateClass:=function(n,q,M)
  G:=GL(n,q);
  cls:=ConjugacyClasses(G);
  return First(cls,c->IsConjugate(G,Representative(c),M*Z(q)^0));
end;

This code uses the boolean-valued function IsConjugate. Note that First(list,f) applies the boolean-valued function f to successive elements of list, and returns the first element of list for which f evaluates to true.

So for example:

gap> LocateClass(2,17,[[1,1],[0,1]]);
[ [ 0*Z(17), Z(17)^8 ], [ Z(17)^0, Z(17)^14 ] ]^G
gap> LocateClass(2,17,[[1,5],[5,1]]);
[ [ Z(17)^15, 0*Z(17) ], [ 0*Z(17), Z(17)^4 ] ]^G
gap> LocateClass(2,17,[[1,5],[5,1]]*Z(17^2));
fail

A bit more on group actions

Let's consider a finite field $k=\mathbb{F}_q$ and a 2 dimensional vector space $V \simeq k^2$ over $k$.

The group G=GL(4,q) acts on $V$ by matrix multiplication.

For a particular q, we can ask GAP for the list of G orbits on elements of V as follows:

gap> G:=GL(2,13);
gap> k:=GF(13);     # "Galois field" = finite field of order 13
gap> V:=k^2;
gap> oo:=OrbitsDomain(G,V);
[ [ [ 0*Z(13), 0*Z(13) ] ], 
  [ [ 0*Z(13), Z(13)^0 ], [ Z(13)^6, 0*Z(13) ], [ Z(13)^7, 0*Z(13) ], 
      [ Z(13)^0, Z(13)^6 ], [ Z(13)^8, 0*Z(13) ], [ Z(13), Z(13)^7 ], 
      [ Z(13), Z(13)^6 ], [ Z(13)^9, 0*Z(13) ], [ Z(13)^2, Z(13)^8 ], 
      [ Z(13)^2, Z(13)^7 ], [ 0*Z(13), Z(13) ], [ Z(13)^2, Z(13)^6 ], 
      ...

      ## there are two orbits...
gap> oo[1];
[ [ 0*Z(13), 0*Z(13) ] ]
gap> Length(oo[2]);
168

Now, the stabilizer of a 1-dimensional subspace is the "upper triangular subgroup", which we can construct as the normalizer of a $13$-sylow subgroup.

A 13-sylow subgroup is equal to

$$\left\{\begin{bmatrix} 1 & x \\ 0 & 1 \end{bmatrix} \mid x \in \mathbb{F}_{13}.\right\}$$

and the normalizer this sylow is $$B=\left\{\begin{bmatrix} a & x \\ 0 & b \end{bmatrix} \mid a,b,x \in \mathbb{F}_{13}, ab \ne 0.\right\}$$

Let's describe $B$ using GAP in two ways:

As normalizer:

gap> u:=[[1,1],[0,1]]*Z(13)^0;
gap> U:=Subgroup(G,[u]);
ActGroup([ [ [ Z(13)^0, Z(13)^0 ], [ 0*Z(13), Z(13)^0 ] ] ])
gap> B:=Normalizer(G,U);
<group of 2x2 matrices over GF(13)>

And as stabilizer:

gap> BB:=Basis(V);
CanonicalBasis( ( GF(13)^2 ) )
gap> H:=Stabilizer(G,BB[2],OnLines);
Group([ [ [ Z(13), 0*Z(13) ], [ 0*Z(13), Z(13)^0 ] ], 
  [ [ Z(13)^0, 0*Z(13) ], [ Z(13)^6, Z(13) ] ] ])
  
gap> H=B;
true

Now, since the action of $G$ on lines in $V$ is transitive, the set of cosets $G/B$ identifies with the set of such lines.

gap> Index(G,B);
14

Thus there are 14 lines in $V$. Since $G$ acts on lines, we get a (transitive) action of $G$ on a set of 14 elements, which we are going to view as the cosets of $B$ in $G$:

gap> P1:=RightCosets(G,B);
[ RightCoset(<group of size 1872 with 2 generators>,
   [ [ Z(13)^0, 0*Z(13) ], [ 0*Z(13), Z(13)^0 ] ]), 
  RightCoset(<group of size 1872 with 2 generators>,
   [ [ Z(13)^6, Z(13)^0 ], [ Z(13)^6, 0*Z(13) ] ]), 
  RightCoset(<group of size 1872 with 2 generators>,
   [ [ 0*Z(13), Z(13)^6 ], [ Z(13)^0, Z(13)^6 ] ]), 
  RightCoset(<group of size 1872 with 2 generators>,
   [ [ Z(13)^2, 0*Z(13) ], [ Z(13)^8, Z(13) ] ]), 
  RightCoset(<group of size 1872 with 2 generators>,
   [ [ Z(13)^10, 0*Z(13) ], [ Z(13)^11, Z(13)^11 ] ]), 
  RightCoset(<group of size 1872 with 2 generators>,
   [ [ Z(13)^2, Z(13)^7 ], [ Z(13)^3, Z(13)^7 ] ]), 
  RightCoset(<group of size 1872 with 2 generators>,
   [ [ Z(13)^5, Z(13)^5 ], [ Z(13)^4, Z(13)^5 ] ]), 
  RightCoset(<group of size 1872 with 2 generators>,
   [ [ Z(13)^8, Z(13)^2 ], [ Z(13), Z(13)^8 ] ]), 
  RightCoset(<group of size 1872 with 2 generators>,
   [ [ 0*Z(13), Z(13)^3 ], [ Z(13)^11, Z(13)^8 ] ]), 
  RightCoset(<group of size 1872 with 2 generators>,
   [ [ Z(13)^8, Z(13)^2 ], [ Z(13)^0, Z(13)^3 ] ]), 
  RightCoset(<group of size 1872 with 2 generators>,
   [ [ Z(13)^7, Z(13)^8 ], [ Z(13)^0, Z(13)^8 ] ]), 
  RightCoset(<group of size 1872 with 2 generators>,
   [ [ Z(13)^0, Z(13)^5 ], [ Z(13)^2, Z(13)^4 ] ]), 
  RightCoset(<group of size 1872 with 2 generators>,
   [ [ 0*Z(13), Z(13)^8 ], [ Z(13)^2, Z(13) ] ]), 
  RightCoset(<group of size 1872 with 2 generators>,
   [ [ Z(13)^0, Z(13) ], [ Z(13)^9, Z(13)^7 ] ]) ]

This action corresponds to a homomorphism $$\operatorname{GL}_2(\mathbb{F}_{13}) \to S_{14}$$ which we can create in GAP as follows:

gap> hom:=ActionHomomorphism(G,P1,OnRight);
gap> Image(hom);
Group([ (3,4,6,10,12,7,5,13,14,9,11,8), (1,2,3)(4,8,5)(7,12,9)(10,14,13) ])

gap> Kernel(hom);
Group([ [ [ Z(13)^3, 0*Z(13) ], [ 0*Z(13), Z(13)^3 ] ], 
  [ [ Z(13)^4, 0*Z(13) ], [ 0*Z(13), Z(13)^4 ] ], 
  [ [ Z(13)^8, 0*Z(13) ], [ 0*Z(13), Z(13)^8 ] ], 
  [ [ Z(13)^9, 0*Z(13) ], [ 0*Z(13), Z(13)^9 ] ] ])

gap> Order(Kernel(hom));    # should be just the scalar matrices!
12