Hi,

I'm trying to form subgroups of the class group and unit group of a
number field.  To get the subgroup of squares in C, I start with

PolynomialRing(ZZ, 'x')
A=-2
B=15
f=x^3 + A*x +B
D=-4*A^3-27*B^2
L.<c> = NumberField(f)
C = L.class_group()
CSq = [n*n for n in C.gens()]

Then, then I run

C.subgroup(CSq)

I get the following error:

Traceback (click to the left of this block for traceback)
...
   executing gensH:=[Fractional ideal class (4, c^2 - c - 1)];

What am I doing wrong?  From looking at archived posts here, it seems
that class groups used to be missing some functionality, but as far as
I can tell, I'm just not using the subgroup command correctly.  I
*was* using sage 4.3.3, but I just upgraded to 4.7, and now I'm
getting a different error.  C.cayley_table() works fine, so I'm
guessing that the class group now has full functionality.  (I haven't
tried anything with the group of units yet).

Ultimately, I don't actually need the subgroup of squares in C (I will
need the 2-torsion subgroup, though), but it seemed like a simple
enough example to start with.  I will also need to form a subgroup of
primes in the group of fractional ideals that divide a certain number,
and then the quotient of that subgroups with the subgroup of its
squares.  I will also need to form a particular subgroup of the group
of units, then the quotient of that with its subgroup of squares.

Thanks,
Zach

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