OFFSET
0,3
FORMULA
a(n) = n*A124427(n). - Joerg Arndt, Dec 04 2010.
E.g.f.: (x+x^2) * exp(x) * exp(exp(x)-1).
EXAMPLE
a(2)= 6 because the equivalence relations on {1,2}: {(1,1), (2,2)}, {(1,1), (2,2), (1,2), (2,1)} contain 6 ordered pairs.
MATHEMATICA
f[list_] := Length[list]^2; Table[Total[Map[f, Level[SetParttions[n], {2}]]], {n, 0, 12}] (* or *)
Range[0, 20]! CoefficientList[Series[(x + x^2)Exp[x] * Exp[Exp[x] - 1], {x, 0, 20}], x]
CROSSREFS
KEYWORD
nonn
AUTHOR
Geoffrey Critzer, Dec 04 2010
STATUS
approved