#15060: The empty graph once again
-------------------------------------------------+-------------------------
Reporter: darij | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-5.12
Component: combinatorics | Resolution:
Keywords: graphs, border cases, bitset, | Merged in:
memleak | Reviewers:
Authors: | Work issues:
Report Upstream: N/A | Commit:
Branch: | Stopgaps:
Dependencies: |
-------------------------------------------------+-------------------------
Comment (by ncohen):
> I noticed the issue when I tried to compute the chromatic polynomial of
an empty graph and got a wrong answer with #14529 applied (and an error
without). It starts by checking whether the graph is connected and if not,
multiplying the characteristic polynomials of the connected components. I
claim that most of the time, connectedness appears together with connected
components, and from the viewpoint of connected components it is much more
natural to have empty objects disconnected.
`O_o`
What is wrong with "a graph is connected if any two vertices are linked by
a path" ? An empty graph can be partitionned in connected component, i.e.
only one : itself.
> If you are saying it's theology, can't we rather output an error instead
of claiming it is "True"?
I would love it to be {{{"ValueError: We don't know whether an empty graph
is connected. We discussed it, and just never agreed on anything"}}}
It would make it a very practical issue rather than a theoretical one, and
emphasize that definitions are mostly what people agree on `:-P`
The same should hold for `Graph({}).is_tree()` of course `:-P`
Nathann
--
Ticket URL: <http://trac.sagemath.org/ticket/15060#comment:4>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
--
You received this message because you are subscribed to the Google Groups
"sage-trac" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To post to this group, send email to [email protected].
Visit this group at http://groups.google.com/group/sage-trac.
For more options, visit https://groups.google.com/groups/opt_out.
