OFFSET
0,3
LINKS
Michael De Vlieger, Table of n, a(n) for n = 0..363
FORMULA
a(n) = n! * Sum_{k=0..n} (n-k+1)^(k-1) * binomial(3*k,n-k)/k!.
MATHEMATICA
Array[#!*Sum[ (# - k + 1)^(k - 1)*Binomial[3 k, # - k]/k!, {k, 0, #}] &, 18, 0] (* Michael De Vlieger, Aug 18 2023 *)
PROG
(PARI) a(n) = n!*sum(k=0, n, (n-k+1)^(k-1)*binomial(3*k, n-k)/k!);
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Aug 17 2023
STATUS
approved