A121678 a(n) = [x^n] (1 + x*(1+x)^n )^(n+1).

1, 2, 9, 52, 425, 4236, 49294, 655096, 9731313, 159114880, 2832245911, 54400757016, 1119436524947, 24532373640334, 569732648555295, 13962373137304496, 359767723241891425, 9715902692094061488

Offset: 0

Author

Paul D. Hanna, Aug 15 2006

Keywords

• nonn

Comments

a(n) is divisible by (n+1): a(n)/(n+1) = A121679(n).

Examples

			At n=5, a(5) = [x^5] (1 + x*(1+x)^5)^6 = 4236, since
(1+x*(1+x)^5)^6 = 1 + 6*x + 45*x^2 + 230*x^3 + 1050*x^4 + 4236*x^5 +...
		

Crossrefs

Cf. A121679; variants: A121673-A121676, A121680.

Programs

PARI

a(n)=sum(k=0,n+1,binomial(n+1,k)*binomial(n*k,n-k))

Formula

a(n) = Sum_{k=0..n+1} C(n+1,k) * C(n*k,n-k).