[GAP Forum] bug with group ring

[Alper Odabaş]

Hi all

If H is a subgroup of G, then R[H] is a subring of R[G]. Similarly, if S is a subring of R, S[G] is a subring of R[G].

for exapmle; 
let R=Z_2 , G=C_3={e,g,g^2} and H={e} . 
A=Z_2C_3 = {0,1,g,g^2,g+g^2,1+g,1+g^2,1+g+g^2}  and B=Z_2H={0,1} then B is a subring A.

but in GAP

gap> G:=Group((1,2,3));;
gap> H:=Subgroup(C,[()]);;
gap> IsSubgroup(G,H);
true
gap> A:=GroupRing(GF(2),G);;
gap> B:=GroupRing(GF(2),H);;
gap> Elements(A);
[ <zero> of ..., (Z(2)^0)*(), (Z(2)^0)*()+(Z(2)^0)*(1,2,3), 
  (Z(2)^0)*()+(Z(2)^0)*(1,2,3)+(Z(2)^0)*(1,3,2), (Z(2)^0)*()+(Z(2)^0)*(1,3,2),
  (Z(2)^0)*(1,2,3), (Z(2)^0)*(1,2,3)+(Z(2)^0)*(1,3,2), (Z(2)^0)*(1,3,2) ]
gap> Elements(B);
[ <zero> of ..., (Z(2)^0)*() ]
gap> IsSubset(A,B);
false
gap> IsIdeal(A,B);
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `IsTwoSidedIdealOp' on 2 arguments calle\
d from
oper( super, sub ) called from
<function>( <arguments> ) called from read-eval-loop
Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
brk>