Opened 8 years ago

# Allow automorphism group of a graph to act on the graph's vertex set

Reported by: Owned by: azi jason, ncohen, rlm major sage-6.4 graph theory rbeezer, ncohen, dcoudert, benjaminfjones N/A

The current implementation of the automorphism group of a graph always relabels the graph in question to use the vertex set {1,2,...,n+1} and then returns the automorphism group based on this labeling and potentially a translation for the labeling as well.

Needless to say that this is time consuming, confusing and also prone to bugs since if one uses integers as labels (starting at 0 for example) the relabeling might go unnoticed.

As it appears from this sage-devel post it is now possible to use arbitrary domains for permutation groups.

Hence, it would be nice to integrate this change into the automorphism_group function as well.

In the long term, I think the right solution is to copy what is done for Galois groups of number fields: depending on a keyword option to automorphism_group(), we should return either the abstract permutation group (as is done now) or a group equipped with an action on the edges and vertices of the graph. David

--- Also what appears to be a 'defect' is that if you have a graph G on say 150 vertices and you want to compute the stabilizer (or some other subgroup..) of Aut(G) and you do

```A = G.automorphism_group()
S = A.stabilizer(v)

```

it happens that you lose the information about the vertices of G. Since S can be a group on a smaller domain and there is no translation from it to the vertices of G

### comment:2 Changed 8 years ago by benjaminfjones

• Description modified (diff)
• Summary changed from Allow automorphism of groups to work on the original vertex set to Allow automorphism group of a graph to act on the graph's vertex set

### comment:3 Changed 8 years ago by azi

• Description modified (diff)

### comment:4 Changed 8 years ago by jason

I posted this to the mailing list, but it's relevant here too:

Until we have proper support for this, you can do:

```sage: g=Graph({'a': 'b', 'b': 'c'})
sage: p,q=g.automorphism_group(translation=True)
sage: pp=PermutationGroup(gap_group=p._gap_(), domain=sorted(q, key=q.get))

sage: pp
Permutation Group with generators [('c','a')]
```

(this also helps you see how to implement it...)

Unfortunately, there is a bug in the stabilizer method when the permutation group has a custom domain, which is now fixed at #13893, which now needs review.

a workaround for the stabilizer bug is:

```sage: pp.subgroup(gap_group=gap.Stabilizer(pp, pp._domain_to_gap['b']))
Subgroup of (Permutation Group with generators [('c','a')]) generated by [('c','a')]
```

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

• Milestone changed from sage-5.11 to sage-5.12

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

• Milestone changed from sage-6.1 to sage-6.2

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

• Milestone changed from sage-6.2 to sage-6.3

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

• Milestone changed from sage-6.3 to sage-6.4
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