OFFSET
1,2
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..80
FORMULA
E.g.f.: G(x)-G(x)/(exp(x)*exp(x^2/2)) where G(x) is the e.g.f. for A006125.
EXAMPLE
a(4) = 26 because we have: * * * *; * * *-* times 6 labelings; *-*-* * times 12 labelings; *-* *-* times 3 labelings; and the complete graph on three nodes union with an isolated node which has 4 labelings. 1+6+12+3+4 = 26.
MATHEMATICA
nn = 17; g = Sum[2^Binomial[n, 2] x^n/n!, {n, 0, nn}]; Drop[Range[0, nn]! CoefficientList[Series[g - g/(Exp[x] Exp[x^2/2]), {x, 0, nn}], x], 1]
CROSSREFS
KEYWORD
nonn
AUTHOR
Geoffrey Critzer, Apr 14 2012
STATUS
approved