Hello,
This looks like a bug in the method incidence_matrix itself
def incidence_matrix(...):
...
cols.sort()
return matrix(cols, sparse=sparse).transpose()
In other words, the matrix is sorted before being returned and there is
no correspondance with anything!
Vincent
PS: BTW, this method is very badly implemented...
On 18/05/15 13:05, David Joyner wrote:
> Hi all:
>
> This issue is undocumented and seems odd to me. I could not find it
> mentioned in sage-support before.
>
> sage: Gamma1 = graphs.CompleteGraph(4); Gamma1
> Complete graph: Graph on 4 vertices
> sage: V1 = Gamma1.vertices(); V1
> [0, 1, 2, 3]
> sage: E1 = Gamma1.edges(); E1
> [(0, 1, None),
> (0, 2, None),
> (0, 3, None),
> (1, 2, None),
> (1, 3, None),
> (2, 3, None)]
> sage: Gamma1.incidence_matrix()
> [-1 -1 -1 0 0 0]
> [ 0 0 1 -1 -1 0]
> [ 0 1 0 0 1 -1]
> [ 1 0 0 1 0 1]
>
> In other words, if you look at the ordering of the edges of Gamma1
> determined by the incidence matrix that Sage returns, the first edge
> (indicated by the first column) is (0,3). However, if you look at the
> ordering of the edges of Gamma1 determined by the edges method, the
> first edge is (0,1).
>
> Maybe not a bug but an unusual feature?
>
> - David Joyner
>
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