davidp wrote:
> Hi,
>
> It seems to me that the show method for DiGraphs is not working
> correctly in Sage 3.4.
>
> EXAMPLE:
> sage: G = DiGraph({1:{2: 1}, 2:{1:1}})
> sage: G.show()
> sage: DiGraph(G.laplacian_matrix()).show()
>
> 1. The first call to show draws the graph with an edge missing.
This seems like a bug.
> 2. The second call produces edges with no arrows.
This is a bug, but probably not for the reason that you think it is.
The bug is that weighted graphs do not show arrows, but should.
> 3. Why should these be different?
Your second graph is a weighted graph with loops. Look at what
G.laplacian_matrix() gives to see why.
What should be the same is DiGraph(G.adjacency_matrix()).show(). Is it?
> 4. In the second call, there should not be loops at each vertex.
Yes, there should be. It's a weighted graph with loops on the vertices,
since the laplacian has nonzero diagonal entries.
Thanks,
Jason
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