A372506 Coefficient of x^n in the expansion of 1 / ( (1-x) * (1-2*x) )^n.

1, 3, 23, 198, 1795, 16758, 159446, 1537308, 14967843, 146833830, 1449054178, 14369723316, 143072565454, 1429331585724, 14320668653580, 143838879376248, 1447883909314851, 14602334949928710, 147518977428892010, 1492559101878005700, 15121898521185194970

Offset

0,2

Links

Table of n, a(n) for n=0..20.

Formula

a(n) = Sum_{k=0..n} binomial(n+k-1,k) * binomial(3*n-1,n-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) * (1-2*x) ).

a(n) ~ (1 + sqrt(3)) * 2^(n - 3/2) * 3^((3*n-1)/2) / sqrt(Pi*n). - Vaclav Kotesovec, May 04 2024

Mathematica

Table[SeriesCoefficient[1/((1 - x)*(1 - 2*x))^n, {x, 0, n}], {n, 0, 20}] (* Vaclav Kotesovec, May 04 2024 *)
Table[Binomial[3*n - 1, n] * Hypergeometric2F1[-n, n, 2*n, -1], {n, 0, 20}] (* Vaclav Kotesovec, May 04 2024 *)

Prog

(PARI) a(n, s=1, t=1, u=1) = sum(k=0, n\s, binomial(t*n+k-1, k)*binomial((t+u+1)*n-(s-1)*k-1, n-s*k));

Crossrefs

Cf. A001700, A069723, A085614, A255688, A372233.

Sequence in context: A331718 A093155 A370105 * A241886 A096649 A370283

Adjacent sequences: A372503 A372504 A372505 * A372507 A372508 A372509

Keyword

nonn

Author

Seiichi Manyama, May 04 2024

Status

approved