#8655: Fix graph genus (broken on multigraphs?)
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Reporter: boothby | Owner: jason, ncohen, rlm
Type: defect | Status: new
Priority: major | Milestone:
Component: graph theory | Keywords:
Author: | Upstream: N/A
Reviewer: | Merged:
Work_issues: |
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Description changed by boothby:
Old description:
> {{{
> sage: G = Graph(multiedges=True)
> sage: G.add_edge(0,1)
> sage: G.add_edge(0,1)
> sage: G.genus()
> 1
> sage: G.to_simple().genus()
> 0
> }}}
>
> Adding parallel edges to a graph never changes the minimal genus. I'm
> concerned that genus() might be entirely broken... but maybe it's just on
> multigraphs? At the very least, removing parallel edges will speed
> things up and fix this bug.
New description:
{{{
sage: G = Graph(multiedges=True)
sage: G.add_edge(0,1)
sage: G.add_edge(0,1)
sage: G.add_edge(0,1)
sage: G.genus()
1
sage: G.to_simple().genus()
0
}}}
Adding parallel edges to a graph never changes the minimal genus. I'm
concerned that genus() might be entirely broken... but maybe it's just on
multigraphs? At the very least, removing parallel edges will speed things
up and fix this bug.
Strangely, the code works correctly on the graph with 2 parallel edges,
but the bug occurs on the graph with 3 parallel edges.
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Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/8655#comment:1>
Sage <http://www.sagemath.org>
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