OFFSET
0,2
FORMULA
a(n) = (1/(n+1)) * Sum_{k=0..n} 3^k * binomial(2*(n+1),k) * binomial(2*(n+1),n-k).
PROG
(PARI) my(N=20, x='x+O('x^N)); Vec(serreverse(x/((1+x)^2*(1+3*x)^2))/x)
(PARI) a(n) = sum(k=0, n, 3^k*binomial(2*(n+1), k)*binomial(2*(n+1), n-k))/(n+1);
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Mar 21 2024
STATUS
approved