Hellooooooooooooo !
 

> for a simple graph, trace_faces()  gives the expected answer for the faces 
> of a planar graph


That's a good news :-D

the face that should exist between just nodes 3,4 and 5  is not found, it's 
> replaced with a face around nodes 1,2,3,4,5.   Any ideas what is wrong, or 
> other ways of getting an accurate list of the faces in a planar graph?
>
What makes you think that the faces returned by Sage are wrong ? A planar 
graph can have different planar embeddings, and so different sets of faces.

If you call .show() with a specific position for the vertices, this may 
result in a planar graph and you may see some faces defined, but there is 
no reason why .trace_faces() should return the faces using the positions 
you defined earlier for the plot !

In the documentation, .trace_faces() tells you that it can either find the 
embedding by itself, or use an "embedding" argument. In you case you did 
not define the embedding, so trace_faces() finds its own.

As, just before, you called .is_planar(set_embedding=True), the embedding 
used by traces_faces() is the one that has been found by is_planar. But as 
you did not give to is_planar() the position of your vertices (and it does 
not accept positions of vertices as an input anyway),then is computes its 
own embedding. That's all.

So unless you think that the faces returned by Sage do not correspond to a 
planar embedding, technically Sage makes no error.

Now, if you want Sage to work with the position of the vertices you used 
for the plot, what you should do is write a short function which transforms 
(x,y) positions for the vertices into a combinatorial embedding. Then, you 
will be able to call .set_embedding() with that embedding, and then 
trace_faces() will give you the result that you expect, i.e. the faces 
defined by your embedding.

And if you do that, please contribute to Sage by submitting your code, as I 
guess this would definitely be useful to others :-)

Good luuuuuuuuuuuuuuuuuuuuuuuuuuuck !

Nathann

 

>
>
> I'm using 'Sage Version 5.13, Release Date: 2013-12-15'
>
> I realize that trace_faces has been deprecated to just faces()  per this 
> discussion: http://trac.sagemath.org/ticket/15551
>
> but when I run 
> S.faces()
>
> I get the error 
> AttributeError: 'Graph' object has no attribute 'faces'
>
>

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