OFFSET
2,2
PROG
(PARI) my(N=20, x='x+O('x^N)); Vec(serlaplace((exp(exp(exp(exp(x)-1)-1)-1)-1)^2/2))
(PARI) T(n, k) = if(k==0, n<=1, sum(j=0, n, stirling(n, j, 2)*T(j, k-1)));
a(n) = sum(k=1, n-1, binomial(n-1, k)*T(k, 4)*T(n-k, 4));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Feb 12 2022
STATUS
approved