OFFSET
0,2
FORMULA
a(n) = Sum_{k=0..floor(n/3)} binomial(3*n+k-1,k) * binomial(2*n,n-3*k).
The g.f. exp( Sum_{k>=1} a(k) * x^k/k ) has integer coefficients and equals (1/x) * Series_Reversion( x / (1+x)^2 * (1-x^3)^3 ). See A369402.
PROG
(PARI) a(n, s=3, t=3, u=2) = sum(k=0, n\s, binomial(t*n+k-1, k)*binomial(u*n, n-s*k));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Feb 12 2024
STATUS
approved