OFFSET
0,1
FORMULA
O.g.f.: 2/(1 + x) + x Product_{i > 0} 1/(1 - a(i) x^i).
a(n) = Sum_t 2^k * (-1)^w where the sum is over all enriched r-trees of size n, k is the number of leaves, and w is the sum of leaves.
MATHEMATICA
a[n_]:=a[n]=2(-1)^n+Sum[Times@@a/@y, {y, IntegerPartitions[n-1]}];
Array[a, 30]
CROSSREFS
KEYWORD
sign
AUTHOR
Gus Wiseman, Mar 21 2018
STATUS
approved