A361037 a(n) = 20*(3*n)!/((2*n)!*(n+2)!).

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

Table of n, a(n) for n=0..21.

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);

Crossrefs

Cf. A000139, A001764, A005809, A007226, A361038, A361039.

Sequence in context: A324494 A344104 A200984 * A299576 A185993 A109051

Adjacent sequences: A361034 A361035 A361036 * A361038 A361039 A361040

Keyword

nonn,easy

Author

Peter Bala, Mar 04 2023

Status

approved