OFFSET
0,2
FORMULA
a(n) = Sum_{k=0..n} Stirling2(n+k, n) * Stirling2(2*n-k, n).
a(n) ~ c * d^n * (n-1)!, where d = 27 / (4*LambertW(-3*exp(-3/2)/2)^2 * (3 + 2*LambertW(-3*exp(-3/2)/2))) = 9.858422414446789720857925020919293523149... and c = 0.28482428628793763109169664913715827647091747... - Vaclav Kotesovec, Dec 28 2021
MATHEMATICA
a[n_] := Coefficient[Series[Product[1/(1 - k*x)^2, {k, 1, n}], {x, 0, n}], x, n]; Array[a, 17, 0] (* Amiram Eldar, Dec 28 2021 *)
Table[Sum[StirlingS2[n + k, n]*StirlingS2[2*n - k, n], {k, 0, n}], {n, 0, 20}] (* Vaclav Kotesovec, Dec 29 2021 *)
PROG
(PARI) a(n) = sum(k=0, n, stirling(n+k, n, 2)*stirling(2*n-k, n, 2));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Dec 27 2021
STATUS
approved