OFFSET
0,2
LINKS
R. W. Robinson, Counting arrangements of bishops, pp. 198-214 of Combinatorial Mathematics IV (Adelaide 1975), Lect. Notes Math., 560 (1976). [The sequence zeta(2k+1).]
FORMULA
From_Reinhard Zumkeller_, Feb 16 2010: (Start)
a(n) = ceiling((((2*n)! / n!)^2) / 2).
From Benedict W. J. Irwin, Jun 05 2016: (Start)
G.f. for a(n)/(n!)^2 : 1/2 + EllipticK(16*x)/Pi, which is the E.g.f for A187535.
G.f. for a(n)/(n!)^3 : 2F2(1/2, 1/2; 1, 1; 16z)/2.
a(n) = n!*A187535(n) = binomial(2*n-1, n-1)*(2*n)!.
(End)
a(n) = A156992(2n,n). - Alois P. Heinz, Apr 30 2017
a(n) ~ asy(2*n-1) where asy(n) = (2*n/e)^n*(18*n + 6 + 1/n)/9. - Peter Luschny, Dec 05 2019
Sum_{n>=0} 1/a(n) = 1 + StruveL(0, 1/2)*Pi/4, where StruveL is the modified Struve function. - Amiram Eldar, Dec 04 2022
MAPLE
For Maple program see A005635.
MATHEMATICA
Table[(((2 n)!/n!)^2)/2, {n, 1, 20}] (* Benedict W. J. Irwin, Jun 05 2016 *)
Table[SeriesCoefficient[Series[1/2 + EllipticK[16 x]/Pi, {x, 0, 20}], n] n! n!, {n, 1, 20}] (* Benedict W. J. Irwin, Jun 05 2016 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Sep 28 2006
EXTENSIONS
a(0)=1 prepended by Alois P. Heinz, Apr 30 2017
STATUS
approved