Opened 8 years ago

Closed 6 years ago

# "subgroups" fails for C1 x C2

Reported by: Owned by: mstreng malb major sage-6.4 group theory abelian group subgroups invfac one bisson Travis Scrimshaw Frédéric Chapoton N/A 1b92c39 (Commits) 1b92c39c04fd82fd3769abbd8059e7dda0de865b

### Description

In Sage 5.7:

```sage: A = AbelianGroup([2,1])
sage: A
Multiplicative Abelian group isomorphic to C2 x C1
sage: A.subgroups()
[Multiplicative Abelian subgroup isomorphic to C2 generated by {f0},
Trivial Abelian subgroup]

sage: B = AbelianGroup([1,2])
sage: B
Multiplicative Abelian group isomorphic to C1 x C2
sage: B.subgroups()
Vectors not LI:  [[0, 0], [0, 1]]
...
ValueError: The given basis vectors must be linearly independent.
```

This last command should work just as well as A.subgroups().

### comment:1 Changed 8 years ago by mstreng

In Sage 5.6, it was still:

```sage: A = AbelianGroup([2,1])
sage: A
Multiplicative Abelian Group isomorphic to C2
sage: A.subgroups()
[Multiplicative Abelian Group isomorphic to C2, which is the subgroup of
Multiplicative Abelian Group isomorphic to C2
generated by [f], Trivial Abelian Group, which is the subgroup of
Multiplicative Abelian Group isomorphic to C2
generated by []]
sage: B = AbelianGroup([1,2])
sage: B
Multiplicative Abelian Group isomorphic to C2
sage: B.subgroups()
[Multiplicative Abelian Group isomorphic to C2, which is the subgroup of
Multiplicative Abelian Group isomorphic to C2
generated by [f], Trivial Abelian Group, which is the subgroup of
Multiplicative Abelian Group isomorphic to C2
generated by []]
```

I think the situation in Sage 5.7 is an improvement, except of course for the failure to compute `B.subgroups()`.

### comment:3 Changed 7 years ago by jdemeyer

• Milestone changed from sage-5.11 to sage-5.12

### comment:4 Changed 7 years ago by vbraun_spam

• Milestone changed from sage-6.1 to sage-6.2

### comment:5 Changed 7 years ago by vbraun_spam

• Milestone changed from sage-6.2 to sage-6.3

### comment:6 Changed 6 years ago by vbraun_spam

• Milestone changed from sage-6.3 to sage-6.4

### comment:7 Changed 6 years ago by tscrim

• Authors set to Travis Scrimshaw
• Branch set to public/groups/fix_subgroups-14196
• Commit set to 1b92c39c04fd82fd3769abbd8059e7dda0de865b
• Component changed from commutative algebra to group theory
• Status changed from new to needs_review

New commits:

 ​1b92c39 `Fix for subgroups of abelian groups whose first part is a trivial group.`

### comment:8 Changed 6 years ago by ncohen

Is it me or is this commit in Sage already ?

Nathann

### comment:9 Changed 6 years ago by ncohen

It seems to be me. And the trac server which seems to say that the diff between this branch and Sage is empty `O_o`

### comment:10 Changed 6 years ago by tscrim

I think it's a trac bug that pops up sporadically.

### comment:11 Changed 6 years ago by chapoton

• Reviewers set to Frédéric Chapoton
• Status changed from needs_review to positive_review

ok, lgtm

Thanks Frederic.

### comment:13 Changed 6 years ago by vbraun

• Branch changed from public/groups/fix_subgroups-14196 to 1b92c39c04fd82fd3769abbd8059e7dda0de865b
• Resolution set to fixed
• Status changed from positive_review to closed
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