OFFSET
0,2
FORMULA
E.g.f.: ( (1/x) * Series_Reversion( x*exp(-x)/(1+x) ) )^2.
E.g.f.: B(x)^2, where B(x) is the e.g.f. of A088690.
a(n) = 2 * n! * Sum_{k=0..n} (n+2)^(k-1) * binomial(n+2,n-k)/k!.
PROG
(PARI) my(N=20, x='x+O('x^N)); Vec(serlaplace((serreverse(x*exp(-x)/(1+x))/x)^2))
(PARI) a(n) = 2*n!*sum(k=0, n, (n+2)^(k-1)*binomial(n+2, n-k)/k!);
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Jan 06 2025
STATUS
approved