#12797: The cut returned by edge_cut of undirected weighted graphs is sometimes
incorrect
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Reporter: hartke | Owner: jason, ncohen, rlm
Type: defect | Status: new
Priority: major | Milestone: sage-6.3
Component: graph theory | Resolution:
Keywords: | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: | Stopgaps:
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Changes (by foosterhof):
* cc: rudi (added)
Comment:
This bug can quite easily be fixed by replacing in the code the part:
{{{
g = self.copy()
for u,v,l in flow_graph.edge_iterator():
if (not use_edge_labels or
(weight(g.edge_label(u,v)) == weight(l))):
g.delete_edge(u,v)
}}}
by:
{{{
g = self.copy()
for u,v,l in flow_graph.edge_iterator():
g.add_edge(v, u) # <-- Added line
if (not use_edge_labels or
(weight(g.edge_label(u,v)) == weight(l))):
g.delete_edge(u,v)
}}}
This is because the usual algorithms search for the vertices reachable by
s in the residual graph. However, the original graph is not a residual
graph, and thus the constructed graph "g" not as well, as it has no back-
edges. Therefore, the partition of vertices is calculated incorrectly, and
therefor the edges exhibiting the cut as well.
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Ticket URL: <http://trac.sagemath.org/ticket/12797#comment:4>
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