This page is devoted to answering some basic questions along the line
“How do I construct … in GAP?” You may view the html source code
for the GAP commands without the output or GAP prompt.
Please send suggestions, additions, corrections to
David Joyner.
This page itself is under construction…
Questions
How
do I … a polynomial? 

How do I find the … of a group
representation? 

How
do I compute an mod m, where A is …? 
Given
a group G, how do I compute … ? 
Answers
gap> G:=Group((1,2)(3,4),(1,2,3)); Group([ (1,2)(3,4), (1,2,3) ])
This is of course a subgroup of the symmetric group S_{4} on 4
letters.
Indeed, this G is in fact the alternating group
on four letters, A_{4}.
By virtue of the fact that the permutations generating G employ
integers less than or equal to 4, this group G
is a subgroup of the symmetric group S_{4} on 4
letters. Some permutation groups have special constructions:
gap> S4:=SymmetricGroup(4); Sym( [ 1 .. 4 ] ) gap> A4:=AlternatingGroup(4); Alt( [ 1 .. 4 ] ) gap> IsSubgroup(S4,G); true gap> IsSubgroup(A4,G); true gap> S3:=SymmetricGroup(3); Sym( [ 1 .. 3 ] ) gap> IsSubgroup(S3,G); false
gap> G:=DihedralGroup(6); gap> Size(G); 6 gap> f:=GeneratorsOfGroup( G ); [ f1, f2 ] gap> f[1]^2; f[2]^3; identity of ... identity of ... gap> f[1]^2= f[2]^3; true
 cyclic group
To construct a cyclic group, you may
construct integers mod n:gap> R:=ZmodnZ( 12); (Integers mod 12) gap> a:=Random(R); ZmodnZObj( 11, 12 ) gap> 4*a; ZmodnZObj( 8, 12 ) gap> b:=Random(R); ZmodnZObj( 9, 12 ) gap> a+b; ZmodnZObj( 8, 12 )
or use the special “CyclicGroup” command
gap> G:=CyclicGroup(12); pc group of size 12 with 3 generators gap> a:=Random(G); f3^2 gap> f:=GeneratorsOfGroup( G ); [ f1, f2, f3 ] gap> f[1]^4; f3 gap> f[1]^12; identity of ...
 conjugacy:
The conjugacy classes of a group G are computed using
the “ConjugacyClasses” command. This is a list
of classes{x^1*g*x  x in G}.gap> G:=SL(2,7); SL(2,7) gap> CG:=ConjugacyClasses(G); [ [ [ 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 ] gap> g:=Representative(CG[3]); Order(g); [ [ 0*Z(7), Z(7)^4 ], [ Z(7)^5, Z(7)^5 ] ] 14 gap> g:=Representative(CG[4]); Order(g); [ [ Z(7)^3, 0*Z(7) ], [ 0*Z(7), Z(7)^3 ] ] 2 gap> g:=Representative(CG[5]); Order(g); [ [ 0*Z(7), Z(7)^3 ], [ Z(7)^0, Z(7)^2 ] ] 7 gap> g:=Representative(CG[6]); Order(g); [ [ 0*Z(7), Z(7)^4 ], [ Z(7)^5, Z(7)^2 ] ] 7 gap>
 presented
To construct a finitely presented group in GAP, use the
“FreeGroup” and “” commands. Here is one example.gap> M12 := MathieuGroup( 12 ); Group([ (1,2,3,4,5,6,7,8,9,10,11), (3,7,11,8)(4,10,5,6), (1,12)(2,11)(3,6)(4,8)(5,9)(7,10) ]) gap> F := FreeGroup( "a", "b", "c" ); free group on the generators [ a, b, c ] gap> words := [ F.1, F.2 ]; [ a, b ] gap> P := PresentationViaCosetTable( M12, F, words ); presentation with 3 gens and 10 rels of total length 97 gap> TzPrintRelators( P ); #I 1. c^2 #I 2. b^4 #I 3. a*c*a*c*a*c #I 4. a*b^2*a*b^2*a*b^2 #I 5. a^11 #I 6. a^2*b*a^2*b^2*a*b^1*a^2*b^1 #I 7. a*b*a^1*b*a^1*b^1*a*b*a^1*b*a^1*b^1 #I 8. a^2*b*a^2*b^2*a^1*b*a^1*b^1*a^1*b^1 #I 9. a*b*a*b*a^2*b^1*a^1*b^1*a*c*b*c #I 10. a^4*b*a^2*b*a^2*c*a*b*a^1*c gap> G := FpGroupPresentation( P ); fp group on the generators [ a, b, c ] gap> RelatorsOfFpGroup( G ); [ c^2, b^4, a*c*a*c*a*c, a*b^2*a*b^2*a*b^2, a^11, a^2*b*a^2*b^2*a*b^1*a^2*b^1, a*b*a^1*b*a^1*b^1*a*b*a^1*b*a^1*b^1, a^2*b*a^2*b^2*a^1*b*a^1*b^1*a^1*b^1, a*b*a*b*a^2*b^1*a^1*b^1*a*c*b*c, a^4*b*a^2*b*a^2*c*a*b*a^1*c ] gap> Size(M12); 95040 gap> Size(G); 95040 gap> IsomorphismGroups(G,M12); ????????
The last command is computationally intensive and requires more
than the default memory allocation of 256M of RAM.Here is another example.
gap> F := FreeGroup( "a", "b"); free group on the generators [ a, b ] gap> G:=F/[F.1^2,F.2^3,F.1*F.2*F.1^(1)*F.2^(1)]; fp group on the generators [ a, b ] gap> Size(G); 6
 rref
The key command for row reduction is “TriangulizeMat”.
The following example illustrates the syntax.gap> M:=[[1,2,3,4,5],[1,2,1,2,1],[1,1,0,0,0]]; [ [ 1, 2, 3, 4, 5 ], [ 1, 2, 1, 2, 1 ], [ 1, 1, 0, 0, 0 ] ] gap> TriangulizeMat(M); gap> M; [ [ 1, 0, 0, 1, 1 ], [ 0, 1, 0, 1, 1 ], [ 0, 0, 1, 1, 2 ] ] gap> Display(M); [ [ 1, 0, 0, 1, 1 ], [ 0, 1, 0, 1, 1 ], [ 0, 0, 1, 1, 2 ] ] gap> M:=Z(3)^0*[[1,2,3,4,5],[1,2,1,2,1],[1,1,0,0,0]]; [ [ Z(3)^0, Z(3), 0*Z(3), Z(3)^0, Z(3) ], [ Z(3)^0, Z(3), Z(3)^0, Z(3), Z(3)^0 ], [ Z(3)^0, Z(3)^0, 0*Z(3), 0*Z(3), 0*Z(3) ] ] gap> TriangulizeMat(M); gap> Display(M); 1 . . 2 1 . 1 . 1 2 . . 1 1 2 gap>
 kernel:
There are different methods for matrices over the integers and
matrices over a field.For integer entries, related commands include
“NullspaceIntMat” and “SolutionNullspaceIntMat”
in section
25.1 “Linear equations over the integers and Integral Matrices”
of the reference manual.gap> M:=[[1,2,3],[4,5,6],[7,8,9]]; [ [ 1, 2, 3 ], [ 4, 5, 6 ], [ 7, 8, 9 ] ] gap> NullspaceIntMat(M); [ [ 1, 2, 1 ] ] gap> SolutionNullspaceIntMat(M,[0,0,1]); [ fail, [ [ 1, 2, 1 ] ] ] gap> SolutionNullspaceIntMat(M,[0,0,0]); [ [ 0, 0, 0 ], [ [ 1, 2, 1 ] ] ] gap> SolutionNullspaceIntMat(M,[1,2,3]); [ [ 1, 0, 0 ], [ [ 1, 2, 1 ] ] ]
Here (0,0,1) is not in the image of M
(under v> v*M) but (0,0,0) and (1,2,3) are.For field entries, related commands include
“NullspaceMat” and “TriangulizedNullspaceMat”
in section
24.6 “Matrices Representing Linear Equations and the Gaussian Algorithm”
of the reference manual.gap> M:=[[1,2,3],[4,5,6],[7,8,9]]; [ [ 1, 2, 3 ], [ 4, 5, 6 ], [ 7, 8, 9 ] ] gap> NullspaceMat(M); [ [ 1, 2, 1 ] ] gap> TriangulizedNullspaceMat(M); [ [ 1, 2, 1 ] ] gap> M:=[[1,2,3,1,1],[4,5,6,1,1],[7,8,9,1,1],[1,2,3,1,1]]; [ [ 1, 2, 3, 1, 1 ], [ 4, 5, 6, 1, 1 ], [ 7, 8, 9, 1, 1 ], [ 1, 2, 3, 1, 1 ] ] gap> NullspaceMat(M); [ [ 1, 2, 1, 0 ], [ 1, 0, 0, 1 ] ] gap> TriangulizedNullspaceMat(M); [ [ 1, 0, 0, 1 ], [ 0, 1, 1/2, 1/2 ] ]
 characteristic polynomial:
Please see section
24.12.1 of the GAP reference manual
for examples of characteristic polynomial of a
square matrix (“CharacteristicPolynomial”) and
section
56.3 for examples of the “characteristic polynomial”
(called a “TracePolynomial”) of an
element of a field extension.
 character:
GAP contains very extensive character theoretic functions
and data libraries (including an interface the character table in the
Atlas).
Here is just one simple example.gap> G:=Group((1,2)(3,4),(1,2,3)); Group([ (1,2)(3,4), (1,2,3) ]) gap> T:=CharacterTable(G); CharacterTable( Alt( [ 1 .. 4 ] ) ) gap> Display(T); CT1 2 2 2 . . 3 1 . 1 1 1a 2a 3a 3b 2P 1a 1a 3b 3a 3P 1a 2a 1a 1a X.1 1 1 1 1 X.2 1 1 A /A X.3 1 1 /A A X.4 3 1 . . A = E(3)^2 = (1ER(3))/2 = 1b3 gap> irr:=Irr(G); [ Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 1, 1, 1, 1 ] ), Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 1, 1, E(3)^2, E(3) ] ), Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 1, 1, E(3), E(3)^2 ] ), Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 3, 1, 0, 0 ] ) ] gap> Display(irr); [ [ 1, 1, 1, 1 ], [ 1, 1, E(3)^2, E(3) ], [ 1, 1, E(3), E(3)^2 ], [ 3, 1, 0, 0 ] ] gap> chi:=irr[2]; gamma:=CG[3]; g:=Representative(gamma); g^chi; Character( CharacterTable( Alt( [ 1 .. 4 ] ) ), [ 1, 1, E(3)^2, E(3) ] ) (1,2,3)^G (1,2,3) E(3)^2
For further details and examples, see chapters
69–
72 of the
GAP reference manual.  brauer:
Just a simple example of what GAP can do here.
To construct a Brauer character table:gap> G:=Group((1,2)(3,4),(1,2,3)); Group([ (1,2)(3,4), (1,2,3) ]) gap> irr:=IrreducibleRepresentations(G,GF(7)); [ [ (1,2)(3,4), (1,2,3) ] > [ [ [ Z(7)^0 ] ], [ [ Z(7)^0 ] ] ], [ (1,2)(3,4), (1,2,3) ] > [ [ [ Z(7)^0 ] ], [ [ Z(7)^4 ] ] ], [ (1,2)(3,4), (1,2,3) ] > [ [ [ Z(7)^0 ] ], [ [ Z(7)^2 ] ] ], [ (1,2)(3,4), (1,2,3) ] > [ [ [ 0*Z(7), Z(7)^3, Z(7)^0 ], [ 0*Z(7), Z(7)^3, 0*Z(7) ], [ Z(7)^0, Z(7)^3, 0*Z(7) ] ], [ [ 0*Z(7), Z(7)^0, 0*Z(7) ], [ 0*Z(7), 0*Z(7), Z(7)^0 ], [ Z(7)^0, 0*Z(7), 0*Z(7) ] ] ] ] gap> brvals := List(irr,chi> List(ConjugacyClasses(G),c> BrauerCharacterValue(Image(chi, Representative(c))))); [ [ 1, 1, 1, 1 ], [ 1, 1, E(3)^2, E(3) ], [ 1, 1, E(3), E(3)^2 ], [ 3, 1, 0, 0 ] ] gap> Display(brvals); [ [ 1, 1, 1, 1 ], [ 1, 1, E(3)^2, E(3) ], [ 1, 1, E(3), E(3)^2 ], [ 3, 1, 0, 0 ] ] gap>
List(ConjugacyClasses(G),c>BrauerCharacterValue(Image(chi, Representative(c)))));
#Display(brvals);
T:=CharacterTable(G);
Display(T);
–>
 polynomial
There are various ways to construct a polynomial in GAP.gap> Pts:=Z(7)^0*[1,2,3]; [ Z(7)^0, Z(7)^2, Z(7) ] gap> Vals:=Z(7)^0*[1,2,6]; [ Z(7)^0, Z(7)^2, Z(7)^3 ] gap> g:=InterpolatedPolynomial(GF(7),Pts,Vals); Z(7)^5*x_1^2+Z(7)
Or:
gap> p:=3;; F:=GF(p);; gap> R:=PolynomialRing(F,["x1","x2"]); PolynomialRing(..., [ x1, x2 ]) gap> vars:=IndeterminatesOfPolynomialRing(R);; gap> x1:=vars[1]; x2:=vars[2]; x1 x2 gap> p:=x1^5x2^5; x1^5x2^5 gap> DivisorsMultivariatePolynomial(p,R); [ x1^4+x1^3*x2+x1^2*x2^2+x1*x2^3+x2^4, x1x2 ]
Or:
gap> x:=X(Rationals); x_1 gap> f:=x+x^2+1; x_1^2+x_1+1 gap> Value(f,[x],[1]); 3
 factor
To factor a polynomial in GAP, there is one command for
univariate polynomials (“Factors”) and another command for
multivariate polynomials (“DivisorsMultivariatePolynomial”).For a factoring a univariate polynomial,
GAP provides only methods over finite fields
and over subfields of cyclotomic fields.
Please see the
examples given in section
64.10 “Polynomial Factorization” for more details.For multivariate polynomials,
a very slow algorithm has been implemented in GAP
and an interface to a very fast algorithm in
Singular
has been implemented for those who have both Singular and
the GAP Singular package
installed. The former of these was
illustrated above in
“polynomial” above.
(Again, the ground field must be a finite field
or a subfields of cyclotomic fields.)
For the latter, please see the example
in the (GAP)Singular manual
FactorsUsingSingularNC.
 roots
There are some situtations where GAP does find the roots
of a polynomial but GAP does not do this generally.
(The roots must generate either a finite field
or a subfield of a cyclotomic field.) However, there is a package called
RadiRoot which must be installed which does help to do this
for polynomials with rational coefficients
(radiroot itself requires other packages to be installed;
please see the webpage for more details).The “Factors” command actually has an option which allows you to
increase the groundfield so that a factorization actually
returns the roots. Please see the
examples given in section
64.10 “Polynomial Factorization” for more details.Here is a second appoach.gap> p:=3; n:=4; F:=GF(p^n); c:=Random(F); r:=2; 3 4 GF(3^4) Z(3^4)^79 2 gap> x:=X(F,1); f:=x^rc*x+c1; x_1 x_1^2+Z(3^4)^39*x_1+Z(3^4)^36 gap> F_f:=FieldExtension( F, f ); AsField( GF(3^4), GF(3^8) ) gap> alpha:=RootOfDefiningPolynomial(F_f); Z(3^4)^36 gap> Value(f,[x],[alpha]); 0*Z(3)
Here is a third. First, enter the following program:
RootOfPolynomial:=function(f,R) local F0,Ff,a; F0:=CoefficientsRing(R); Ff:=FieldExtension(F0,f); a:=RootOfDefiningPolynomial(Ff); return a; end;
Here’s how this can be used to find a root:
gap> F:=Rationals; Rationals gap> x:=X(F,1); f:=x^2+x+1; x_1 x_1^2+x_1+1 gap> R:=PolynomialRing( F, [ x ]); PolynomialRing(..., [ x_1 ]) gap> a:=RootOfPolynomial(f,R); E(3) gap> # check: gap> Value(f,[x],[a]); 0
Related links:
 In the GAP Forum:
Hensel lifting discussion.  In the manual,
Galois groups.
 In the GAP Forum:
 evaluate:
The relevant command is “Value”. There are several examples already on
this page. For others, please see the examples given in section
64.7 Multivariate polynomials of the manual.
For sparse uivariate polynomials, there is also the command
“ValuePol” in section
23.6 of the manual.
 integer power
This is easy and intuitive:gap> a:=1000; n:=100000; m:=123; 1000 100000 123 gap> a^n mod m; 1
 matrix power:
This too is easy and intuitive:gap> A:=[[1,2],[3,4]]; n:=100000; m:=123; [ [ 1, 2 ], [ 3, 4 ] ] 100000 123 gap> A^n mod m; [ [ 1, 41 ], [ 0, 1 ] ]
 polynomial power
GAP allows you to do arithmetic over the polynomial
ring R[x], where R = Z/nZ (where n is a positive integer).
Here’s an example.gap> Z4:=ZmodnZ(4); (Integers mod 4) gap> R:=UnivariatePolynomialRing(Z4,1); PolynomialRing(..., [ x ]) gap> x:=IndeterminatesOfPolynomialRing(R)[1]; x gap> I:=TwoSidedIdealByGenerators( R,[x^8x^0]); twosided ideal in PolynomialRing(..., [ x ]), (1 generators) gap> gen:=x^8x^0; x8ZmodnZObj(1,4) gap> QuotientRemainder(R,x^8,gen); [ ZmodnZObj(1,4), ZmodnZObj(1,4) ] gap> QuotientRemainder(R,x^15,gen); [ x^7, x^7 ] gap> QuotientRemainder(R,x^15+x^8,gen); [ x^7+ZmodnZObj(1,4), x^7+ZmodnZObj(1,4) ] gap> PowerMod( R, x+x^0, 15, gen ); ZmodnZObj(0,4) gap> PowerMod( R, x, 15, gen ); x^7
 Groebner basis
GAP’s Groebner bases algorithms are relatively slow
and are included mostly for simple examples and for
teaching purposes. However, a GAP interface to a very
fast algorithm in Singular
has been implemented for those who have both Singular and
the
GAP Singular package
installed. The former of these is
illustrated in section
64.17 Groebner bases of the GAP manual.
For the latter, please see the example
in the (GAP)Singular manual
GroebnerBasis.
 normal subgroup:
Here is an example:gap> G := AlternatingGroup( 5 ); Group( (1,2,5), (2,3,5), (3,4,5) ) gap> normal := NormalSubgroups( G ); [ Subgroup( Group( (1,2,5), (2,3,5), (3,4,5) ), [ ] ), Subgroup( Group( (1,2,5), (2,3,5), (3,4,5) ), [ (1,2)(3,4), (1,3)(4,5), (1,4)(2,3) ] ) ]
Related links:
 Please see Volkmar Felsch’s
GAP Forum response to a related question.  The
xgap package displays subgroup lattices graphically.
 Please see Volkmar Felsch’s
 abelian subgroup
One idea to compute all the abelian subgroups is to compute all the
subgroups then “filter” out the abelian ones.
Here is an illustration, taked from a
GAP Forum response Volkmar Felsch.gap> G := AlternatingGroup( 5 ); Group( (1,2,5), (2,3,5), (3,4,5) ) gap> classes := ConjugacyClassesSubgroups( G ); [ ConjugacyClassSubgroups( Group( (1,2,5), (2,3,5), (3,4,5) ), Subgroup( Group( (1,2,5), (2,3,5), (3,4,5) ), [ ] ) ), ConjugacyClassSubgroups( Group( (1,2,5), (2,3,5), (3,4,5) ), Subgroup( Group( (1,2,5), (2,3,5), (3,4,5) ), [ (2,3)(4,5) ] ) ), ConjugacyClassSubgroups( Group( (1,2,5), (2,3,5), (3,4,5) ), Subgroup( Group( (1,2,5), (2,3,5), (3,4,5) ), [ (3,4,5) ] ) ), ConjugacyClassSubgroups( Group( (1,2,5), (2,3,5), (3,4,5) ), Subgroup( Group( (1,2,5), (2,3,5), (3,4,5) ), [ (2,3)(4,5), (2,4)(3,5) ] ) ), ConjugacyClassSubgroups( Group( (1,2,5), (2,3,5), (3,4,5) ), Subgroup( Group( (1,2,5), (2,3,5), (3,4,5) ), [ (1,2,3,4,5) ] ) ), ConjugacyClassSubgroups( Group( (1,2,5), (2,3,5), (3,4,5) ), Subgroup( Group( (1,2,5), (2,3,5), (3,4,5) ), [ (3,4,5), (1,2)(4,5) ] ) ), ConjugacyClassSubgroups( Group( (1,2,5), (2,3,5), (3,4,5) ), Subgroup( Group( (1,2,5), (2,3,5), (3,4,5) ), [ (1,2,3,4,5), (2,5)(3,4) ] ) ), ConjugacyClassSubgroups( Group( (1,2,5), (2,3,5), (3,4,5) ), Subgroup( Group( (1,2,5), (2,3,5), (3,4,5) ), [ (2,3)(4,5), (2,4)(3,5), (3,4,5) ] ) ), ConjugacyClassSubgroups( Group( (1,2,5), (2,3,5), (3,4,5) ), Group( (1,2,5), (2,3,5), (3,4,5) ) ) ] gap> cl := classes[4]; ConjugacyClassSubgroups( Group( (1,2,5), (2,3,5), (3,4,5) ), Subgroup( Group( (1,2,5), (2,3,5), (3,4,5) ), [ (2,3)(4,5), (2,4)(3,5) ] ) ) gap> length := Size( cl ); 5 gap> rep := Representative( cl ); Subgroup( Group( (1,2,5), (2,3,5), (3,4,5) ), [ (2,3)(4,5), (2,4)(3,5) ] ) gap> order := Size( rep ); 4 gap> IsAbelian( rep ); true gap> abel := Filtered( classes, cl > IsAbelian( Representative( cl ) ) ); [ ConjugacyClassSubgroups( Group( (1,2,5), (2,3,5), (3,4,5) ), Subgroup( Group( (1,2,5), (2,3,5), (3,4,5) ), [ ] ) ), ConjugacyClassSubgroups( Group( (1,2,5), (2,3,5), (3,4,5) ), Subgroup( Group( (1,2,5), (2,3,5), (3,4,5) ), [ (2,3)(4,5) ] ) ), ConjugacyClassSubgroups( Group( (1,2,5), (2,3,5), (3,4,5) ), Subgroup( Group( (1,2,5), (2,3,5), (3,4,5) ), [ (3,4,5) ] ) ), ConjugacyClassSubgroups( Group( (1,2,5), (2,3,5), (3,4,5) ), Subgroup( Group( (1,2,5), (2,3,5), (3,4,5) ), [ (2,3)(4,5), (2,4)(3,5) ] ) ), ConjugacyClassSubgroups( Group( (1,2,5), (2,3,5), (3,4,5) ), Subgroup( Group( (1,2,5), (2,3,5), (3,4,5) ), [ (1,2,3,4,5) ] ) ) ]
 homology
This depends on how the group is given. For example, suppose that
G is a permutation group with generators genG and
H is a permutation group with generators genH. To find a
homomorphism from G to H, one may use the
“GroupHomomorphismByImages” or “GroupHomomorphismByImagesNC”
commands. For examples of the syntax, please see
section
38.1 Creating Group Homomorphisms.Here’s an illustration of how to convert a finitely presented
group into a permutation group.gap> p:=7; 7 gap> G:=PSL(2,p); Group([ (3,7,5)(4,8,6), (1,2,6)(3,4,8) ]) gap> H:=SchurCover(G); fp group of size 336 on the generators [ f1, f2, f3 ] gap> iso:=IsomorphismPermGroup(H); [ f1, f2, f3 ] > [ (1,2,4,3)(5,9,7,10)(6,11,8,12)(13,14,15,16), (2,5,6)(3,7,8)(11,13,14)(12,15,16), (1,4)(2,3)(5,7)(6,8)(9,10)(11,12)(13, 15)(14,16) ] gap> H0:=Image(iso); # 2cover of PSL2 Group([ (1,2,4,3)(5,9,7,10)(6,11,8,12)(13,14,15,16), (2,5,6)(3,7,8)(11,13,14)(12,15,16), (1,4)(2,3)(5,7)(6,8)(9,10)(11,12)(13, 15)(14,16) ]) gap> IdGroup(H0); [ 336, 114 ] gap> IdGroup(SL(2,7)); [ 336, 114 ] gap>
 semidirect product(Contributed by Nilo de Roock):
As you can easily verify, D8 is isomorphic to C2:C4. Or in GAP…N:=CyclicGroup(IsPermGroup,4); G:=CyclicGroup(IsPermGroup,2); AutN:=AutomorphismGroup(N); f:=GroupHomomorphismByImages(G,AutN,GeneratorsOfGroup(G),[Elements(AutN)[2]]); NG:=SemidirectProduct(G,f,N);
Verify with
StructureDescription(NG);
 semidirect products(Contributed by Nilo de Roock):
The following shows how to construct all nonabelian groups
of order 12 as semidirect products. These products are not
trivial yet small enough to verify by hand.#D12 = (C2 x C2) : C3 G1:=CyclicGroup(IsPermGroup,2); G2:=CyclicGroup(IsPermGroup,2); G:=DirectProduct(G1,G2); N:=CyclicGroup(IsPermGroup,3); AutN:=AutomorphismGroup(N); f:=GroupHomomorphismByImages(G,AutN,[Elements(G)[1],Elements(G)[2],Elements(G)[3],Elements(G)[4]],[Elements(AutN)[1],Elements(AutN)[2],Elements(AutN)[1],Elements(AutN)[2]]); NG:=SemidirectProduct(G,f,N); Print(str(NG)); Print("\n");
#T = C4 : C3 G:=CyclicGroup(IsPermGroup,4); N:=CyclicGroup(IsPermGroup,3); AutN:=AutomorphismGroup(N); f:=GroupHomomorphismByImages(G,AutN,[Elements(G)[1],Elements(G)[2],Elements(G)[3],Elements(G)[4]],[Elements(AutN)[1],Elements(AutN)[2],Elements(AutN)[1],Elements(AutN)[2]]); NG:=SemidirectProduct(G,f,N); Print(str(NG)); Print("\n");
#A4 = C3 : (C2 x C2) G:=CyclicGroup(IsPermGroup,3); N1:=CyclicGroup(IsPermGroup,2); N2:=CyclicGroup(IsPermGroup,2); N:=DirectProduct(G1,G2); AutN:=AutomorphismGroup(N); f:=GroupHomomorphismByImages(G,AutN,[Elements(G)[1],Elements(G)[2],Elements(G)[3]],[Elements(AutN)[1],Elements(AutN)[4],Elements(AutN)[5]]); NG:=SemidirectProduct(G,f,N); Print(str(NG)); Print("\n");
 cohomology
GAP will compute the Schur multiplier
H^{2}(G,C) using the
“AbelianInvariantsMultiplier” command.
Here is an example showing how to find H^{2}(A_{5},C),
where A_{5} is the alternating group on 5 letters.gap> A5:=AlternatingGroup(5); Alt( [ 1 .. 5 ] ) gap> AbelianInvariantsMultiplier(A5); [ 2 ]
So, H^{2}(A_{5},C) is Z/2Z.
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