#12134: is_planar(set_pos=True) doesn't work with small graphs
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Reporter: brunellus | Owner: jason, ncohen, rlm
Type: defect | Status: new
Priority: major | Milestone: sage-4.8
Component: graph theory | Keywords:
Work_issues: | Upstream: N/A
Reviewer: | Author:
Merged: | Dependencies:
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{{{
sage: graphs.PathGraph(2).is_planar(set_pos=True)
Traceback (click to the left of this block for traceback)
...
NotImplementedError: _triangulate() only accepts graphs with more than 2
vertices as input.
}}}
It is interesting that _triangulate raises NotImplementedError. The usual
definition of a triangulation says something like "maximal planar graph",
so it seems like PathGraph(2) is already triangulated, but if you ask
[http://mathworld.wolfram.com/TriangulatedGraph.html Mathematica] or
[http://oeis.org/A000109 OEIS], they don't accept this: according to this
source there aren't any triangulated graphs on two vertices, so
_triangulate should scream "This can't be done" and is_planar should avoid
use _triangulate in this scenario, because there is no doubt that
PathGraph(2) is planar.
The error is only present when set_pos=True. Also, notice that
{{{
sage: graphs.PathGraph(1).is_planar(set_pos=True)
True
}}}
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Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/12134>
Sage <http://www.sagemath.org>
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