OFFSET
1,2
COMMENTS
a(n) is odd if and only if n = 2^i-1 for i in {1, 2, 3, ...} = A000027.
Form an array with m(1,n) = n*(n+1)/2, m(n,1) = n*(n-1)+1, and m(i,j) = m(i,j-1) + m(i-1,j); A000217 in the top row, A002061 in the first column, A086514 in the second column. Then on the diagonal m(n,n) = a(n). - J. M. Bergot, May 02 2013
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..1000
S. B. Ekhad and D. Zeilberger, Computational and Theoretical Challenges on Counting Solid Standard Young Tableaux, arXiv:1202.6229 [math.CO], 2012.
Wikipedia, Young tableau.
FORMULA
a(n) = 2*(2*n-1)^2/((n+1)*(2*n-3)) * a(n-1) for n>1; a(1) = 1.
a(n) = [x^n] x*(1 + 2*x)/(1 - x)^(n+2). - Ilya Gutkovskiy, Oct 12 2017
Sum_{n>=1} 1/a(n) = 1/6 + G + 13*Pi/(36*sqrt(3)) - Pi*log(2+sqrt(3))/8, where G is Catalan's constant (A006752). - Amiram Eldar, Mar 06 2022
From Stefano Spezia, Mar 29 2023: (Start)
O.g.f.: 1 + (3 - 3*sqrt(1 - 4*x) - 8*x)/(2*x*sqrt(1 - 4*x)).
E.g.f.: 1 + exp(2*x)*(3*I_1(2*x) - I_0(2*x)), where I_n(x) is the modified Bessel function of the first kind.
a(n) ~ 2^(1+2*n)/sqrt(n*Pi). (End)
MAPLE
a:= proc(n) option remember;
`if`(n<2, n, 2*(2*n-1)^2*a(n-1)/((n+1)*(2*n-3)))
end:
seq(a(n), n=1..30);
MATHEMATICA
a[n_]:= a[n] = If[n<2, n, 2*(2*n-1)^2*a[n-1]/((n+1)*(2*n-3))]; Array[a, 30] (* Jean-François Alcover, Aug 14 2017, translated from Maple *)
PROG
(PARI) a(n) = (2*n-1) * binomial(2*n, n)/(n+1); \\ Michel Marcus, Mar 06 2022
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Jul 30 2012
STATUS
approved