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A361037
a(n) = 20*(3*n)!/((2*n)!*(n+2)!).
3
10, 10, 25, 84, 330, 1430, 6630, 32300, 163438, 852150, 4552275, 24812400, 137547000, 773564328, 4405019090, 25357898940, 147375745990, 863805209750, 5101386767295, 30332569967700, 181465130121450, 1091677288630950
OFFSET
0,1
COMMENTS
Gessel (1992) introduced sequences {b(r,n): n >= 0} of super ballot numbers defined by b(r,n) = J(r) * (2*n)!/(n!*(n + r + 1)!), r = 0,1,2,..., where J(r) = (2*r + 2)!/(2*(r + 1)!) = (2^r)*Product_{j = 0..r} (2*j + 1) is chosen so that these numbers are always integers. The sequence {b(1,n) : n >= 0} is A000108, the sequence of Catalan numbers. See A135573 for a table of these generalized Catalan numbers.
We carry out an analogous construction using the numbers B(n) = A005809(n) = binomial(3*n,n) = (3*n)!/((2*n)!*n!) in place of the central binomial numbers. We define B(r,n), r = 0,1,2, ..., by B(r,n) = F(r) * (3*n)!/((2*n)!*(n + r + 1)!), where F(r) is the minimal choice to produce integer values for these quantities for all n. This sequence is the case r = 1. See A007226 (r = 0), A361038 (r = 2) and A361039 (r = 3).
LINKS
Ira M. Gessel, Super ballot numbers, J. Symbolic Comp., 14 (1992), 179-194.
FORMULA
a(n) = 10*binomial(3*n,n) - 7*binomial(3*n,n+1) + binomial(3*n,n+2). Thus a(n) is an integer.
P-recursive: 2*(n + 2)(2*n - 1)*a(n) = 3*(3*n - 1)*(3*n - 2)*a(n-1) with a(0) = 10.
a(n) ~ (27/4)^n * 10*sqrt(3/Pi)/n^(5/2).
The o.g.f. satisfies the differential equation
x^2*(27*x - 4)*A''(x) + 2*x*(27*x - 5)*A'(x) + 2*(3*x + 2)*A(x) - 40 = 0, with A(0) = 10 and A'(0) = 10.
MAPLE
seq( 20*(3*n)!/((2*n)!*(n+2)!), n = 0..20);
MATHEMATICA
Table[20 (3n)!/((2n)!(n+2)!), {n, 0, 30}] (* Harvey P. Dale, Aug 05 2024 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Peter Bala, Mar 04 2023
STATUS
approved