OFFSET
1,4
LINKS
Eric Weisstein's World of Mathematics, Chromatic Polynomial.
Eric Weisstein's World of Mathematics, Chromatically Unique Graph.
PROG
(Sage)
def A137567(n):
d = {}
for g in graphs(n):
c = g.chromatic_polynomial()
d[c] = 1 if c not in d else d[c]+1
return sum(d.values()) - d.values().count(1) # D. S. McNeil, Jan 02 2011
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Eric W. Weisstein, Jan 26 2008
EXTENSIONS
a(9) from Eric W. Weisstein, Apr 07 2008
a(8) and a(9) corrected by Eric W. Weisstein, Jan 02 2011
a(10) from Eric W. Weisstein, Jun 09 2014
STATUS
approved