OFFSET
0,2
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
D-finite with recurrence 5*n*(n-1)*a(n) +18*(n-1)*(n-3)*a(n-1) +12*(-45*n^2+90*n-22)*a(n-2) -216*(3*n-7)*(3*n-8)*a(n-3)=0. - R. J. Mathar, Oct 24 2024
MAPLE
A372506 := proc(n)
add(binomial(n+k-1, k)*binomial(3*n-1, n-k), k=0..n) ;
end proc:
seq(A372506(n), n=0..80) ; # R. J. Mathar, Oct 24 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
KEYWORD
nonn
AUTHOR
Seiichi Manyama, May 04 2024
STATUS
approved