OFFSET
0,3
FORMULA
a(n) = Sum_{k=0..n} C(n,k) * C((n+1)*k,n-k).
EXAMPLE
At n=4, a(4) = [x^4] (1 + x*(1+x)^5 )^4 = 371, since
(1 + x*(1+x)^5 )^4 = 1 + 4*x + 26*x^2 + 104*x^3 + 371*x^4 +...
MATHEMATICA
Table[Sum[Binomial[n, k] * Binomial[(n+1)*k, n-k], {k, 0, n}], {n, 0, 20}] (* Vaclav Kotesovec, Jun 12 2015 *)
PROG
(PARI) a(n)=sum(k=0, n, binomial(n, k)*binomial((n+1)*k, n-k))
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Aug 15 2006
STATUS
approved