OFFSET
0,2
FORMULA
G.f. A(x)=x*B'(x)/B(x)+x*(1/x-1/(B(x))', B(x)=x(1+B(x))^4.
a(n) = Sum_{k=0..n+1} C(n+1,k)*C(3*n,k).
PROG
(Maxima)
a(n):=sum(binomial(n+1, k)*binomial(3*n, k), k, 0, n+1);
(PARI) a(n) = binomial(4*n+1, n+1); \\ Michel Marcus, Jun 15 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Vladimir Kruchinin, Jun 15 2020
STATUS
approved