[GAP Forum] working with GroupRings

[tkohl at math.bu.edu]

Dear forum,

This has gotten me part way to what I'm looking for.

(Many thanks Frank.)

But I'm running into a different problem now.

Basically, if one has, for example,

a:=Indeterminate(Rationals,"r");
K:=AlgebraicExtension(Rationals,r^2-2)
S3:=SymmetricGroup(3);
KS3:=GroupRing(K, S3);
emb:=Embedding(S3,KS3);

then I would like to be able to view KS3 as
a module over Q, so that I can do something
like this

H:=Subspace(KD3,[(1,2)^emb+(1,2,3)^emb, (1,3,2)^emb+(1,3)^emb ]);

so that H is the Q-span of { (1,2)+(1,2,3) , (1,3,2)+(1,3) }
with the ultimate goal of being able to multiply elements
of H and represent them with respect to this basis.

[I'm basically looking at Q-subalgebras of KG.]

Also, as an aside, the GaloisGroup() function seems
not to be working. It gives the "no method found" error
if I try to do GaloisGroup(K).  [I'm using 4r8.]

Thanks.

	-T









On Thu, 26 Oct 2017, Frank Lübeck wrote:

> 
> Dear Tim, dear Forum,
> 
> How about using iterated algebraic extensions for the ground field? Even
> then the input of elements is not too nice because GAP does not apply
> implicit embeddings of elements from subfield or of the group into the group
> ring. This works:
> 
> w := Indeterminate(Rationals,"w");
> f1 := AlgebraicExtension(Rationals, w^3-2, "w");
> z := Indeterminate(f1,"z");
> F := AlgebraicExtension(f1, z^2+z+1, "z");
> w := RootOfDefiningPolynomial(f1)*One(F);
> z := RootOfDefiningPolynomial(F);
> w in F;
> z in F;
> S3 := SymmetricGroup(3);
> FS3 := GroupRing(F,S3);
> emb := Embedding(S3, FS3);
> e := function(r) return r*One(f1)*One(F); end;
> x := ((e(-4)*w^2-e(4/3)*w+e(3/2))*z+(2*w^2+2*w-e(2/3)))*(1,2)^emb;
> x + x^2;
> y := (z*(1,2)^emb)^2;
> y = (-z-e(1))*()^emb;
> 
> 
> Best regards,
>   Frank
> 
> On Wed, Oct 25, 2017 at 06:00:57PM -0400, tkohl at math.bu.edu wrote:
> > 
> > Dear Forum members,
> > 
> > This is somewhat related to a question I asked a while
> > back about GroupRings. My question is somewhat general,
> > but I will try to be as brief as possible.
> > 
> > I am trying to construct the group ring  Q[w,z]S_3 where
> > Q[w,z] is the field extension of Q obtained by adjoining w,z where
> > w^3=2 and z is a primitive cube root of unity.
> > 
> > The method I am using is this:
> > 
> > z:=Indeterminate(Rationals,"z");
> > w:=Indeterminate(Rationals,"w");
> > R:=PolynomialRing(Rationals,["z","w"]);
> > I:=Ideal(R,[z^2+z+1,w^3-2]);
> > F:=R/I;
> > S3:=SymmetricGroup(3)
> > FS3:=GroupRing(F,S3);
> > 
> > so far so good.
> > 
> > One initial thing I notice is this:
> > 
> > gap> BF:=BasisVectors(Basis(F));
> > [ (1), (w), (w2), (z), (zw), (zw2) ]
> > 
> > which I can understand corresponds to the ideals 1+I, w+I, w^2+I, etc.
> > but I am not sure how to actually construct expressions by hand.
> > 
> > [That (zw2) is the representative instead of (z*w^2) is a bit jarring too, but that's
> > not the biggest issue.]
> > 
> > i.e. This does not work
> > 
> > gap> (w) in F;
> > false
> > 
> > although if I do 
> > 
> > gap> BF[2] in F
> > 
> > then, of course, it is true. 
> > Q1) How can I specify elements of F without having to refer to the literal list returned
> > from BasisVectors(Basis(F))  ?
> > 
> > Once I'm past this hurdle, I still want to work with elements of FS3 by taking linear
> > combinations of group elements and elements of F. 
> > 
> > Q2) I want to be able to do something like this:
> > 
> > (z*(1,2))*(z*(1,2)) 
> > 
> > and have it give me (-z-1)*()
> > 
> > I know I need to use One(F) or One(FS3) in these expressions, but everything I have tried
> > ends up triggering
> > 
> > "Error, no method found! For debugging hints type ?Recovery from NoMethodFound"
> > 
> > Q3) Alternately, is there a way (like in Maple) to symbolically manipulate a polynomial
> > expression, for example
> > 
> > algsubs(z^2+z+1=0,z^4+z5)
> > 
> > and yield z+z^2? 
> > 
> > (i.e. Forget about using a quotient ring and instead apply some regular expression
> > to 'mod out' by the relations w^3=2 and z^2+z+1=0.)
> > 
> > Pardon the length of my question, and thanks in advance for any assistance.
> > The main reason I'm using GAP in this instance is that Maple's grouptheory and
> > non-commuting variables infrastructure didn't work.
> > 
> > Thanks.
> > 
> > 	-Tim K.
> > 
> > 
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