Since there are no proper colorings of graphs with loops, their chromatic polynomial should be zero.
SageMath 8.6 however seems to ignore the loops and returns a nonzero chromatic polynomial for the graph on one vertex with one loop edge.
sage: Graph([[1, 1]], multiedges=True, loops=True).chromatic_polynomial()
x
This indeed looks like a bug.
As a workaround, test whether the graph has loops before computing the chromatic polynomial.
sage: G = Graph([[1, 1]], multiedges=True, loops=True)
sage: G.has_loops()
True
Fixing the bug should just amount to special-casing graphs with loops (testing for loops as above) and including a doctest (to prevent the bug from reappearing).
Asked: 2019-06-05 14:16:04 +0100
Seen: 285 times
Last updated: Jun 06 '19
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This indeed looks like a bug. Thanks for reporting!
Would you like to open a ticket for it on the Sage Trac server?
Would you like to suggest a fix? The relevant file is
(If not, I can take care of it.)
Now tracked at