OFFSET
0,3
COMMENTS
a(n) is the number of ways to select an ordered pair from each equivalence class in each equivalence relation on {1,2,...,n}. - Geoffrey Critzer, Oct 03 2011
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..500
FORMULA
E.g.f.: exp(exp(x)*(x+x^2)).
MAPLE
a:= proc(n) option remember; `if`(n=0, 1,
add(binomial(n-1, j-1)*j^2*a(n-j), j=1..n))
end:
seq(a(n), n=0..25); # Alois P. Heinz, Mar 30 2016
MATHEMATICA
Range[0, 20]! CoefficientList[Series[Exp[Exp[x](x+x^2)], {x, 0, 20}], x]
Table[Sum[BellY[n, k, Range[n]^2], {k, 0, n}], {n, 0, 20}] (* Vladimir Reshetnikov, Nov 09 2016 *)
PROG
(PARI)
N=33; x='x+O('x^N);
egf=exp(x*(1+x)*exp(x));
Vec(serlaplace(egf))
/* Joerg Arndt, Sep 15 2012 */
CROSSREFS
KEYWORD
nonn
AUTHOR
STATUS
approved