I am somewhat confused by the output from the AbelianInvariants function,
as applied to a finitely generated abelian group. I am using GAP 3.2
with upgrades 1 and 2, on a DEC Ultrix workstation, if it matters. In the
example below, I hoped to get the output [0 0], corresponding to the fact
that the abelianization of the free group on two letters ("a" and "b" below)
is Z+Z. Have I misused the function or misinterpreted the output?

Daniel Ruberman
ruberman@binah.cc.brandeis.edu

gap> a:=AbstractGenerator("a");
a
gap> b:=AbstractGenerator("b");
b
gap> G:=Group(a,b);
Group( a, b )
gap> G.relators:=[];
[  ]
gap> H:=CommutatorFactorGroup(G);
Group( c.1, c.2 )
gap> H.relators;
[ c.1^-1*c.2^-1*c.1*c.2 ]
gap> AbelianInvariants(H);
[ 0 ]