#8372: split up incidence_matrix() over graph.py and digraph.py
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Reporter: mvngu | Owner: rlm
Type: enhancement | Status: needs_work
Priority: major | Milestone: sage-4.3.4
Component: graph theory | Keywords:
Author: Minh Van Nguyen | Upstream: N/A
Reviewer: | Merged:
Work_issues: |
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Comment(by rbeezer):
Hi Minh,
Makes sense to wait on this (and it was no trouble to have a look
inpreparation for the eventual review).
I should have been clearer - my hesitations on a "2" for a loop is *only*
for the case of a digraph. It make abundant good sense for an undirected
graph.
Suppose a digraph only allows for at most a single directed edge between
any pair of vertices (ie, no multiple directed edges). Then shouldn't a
loop have a head and a tail and contribute a +1 and a -1 there? I agree
totally that this is a loss of information, since we can't recover the
loop from the matrix. But I also prefer that these matrices have nice
algebraic properties (like constant row sum, or constant column sums), so
I don't view them totally as simply carriers of enough information to
reconstruct the graph. I can see both sides of the argument.
I'll get back to this once you are ready to return to it.
Rob
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Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/8372#comment:5>
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