#14657: set_embedding fails for multigraphs
----------------------------+-----------------------------------------------
Reporter: nvcleemp | Owner: jason, ncohen, rlm
Type: defect | Status: new
Priority: minor | Milestone: sage-5.10
Component: graph theory | Keywords:
Work issues: | Report Upstream: N/A
Reviewers: | Authors:
Merged in: | Dependencies:
Stopgaps: |
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If you try to set the embedding for a multigraph, you will get an error if
you specify all edges. If you bundle each multi-edge into a single edge it
works. The following code, e.g., gives an error:
{{{
h = Graph({1:[2], 2:[3,3], 3:[]})
h.set_embedding({1:[2], 2:[1,3,3], 3:[2,2]})
}}}
The cause of this is the following lines in the method
`check_embedding_validity`:
{{{
if len(embedding[v]) != len(self.neighbors(v)):
return False
}}}
A multigraph returns each neighbour only once, and not the number of times
that there are edges between these two vertices.
Is this the desired behaviour? If so, then maybe some documentation needs
to be added to warn the user that this will not work. If not, how do we
change it? Should the method `neighbors` return the 'multiset' of
neighbours and not just the 'set'?
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/14657>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
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