OFFSET
0,5
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..200
P. Flajolet and M. Noy, Analytic combinatorics of non-crossing configurations, Discrete Math., 204, 203-229, 1999.
Emanuele Munarini, Shifting Property for Riordan, Sheffer and Connection Constants Matrices, Journal of Integer Sequences, Vol. 20 (2017), Article 17.8.2.
M. Noy, Enumeration of noncrossing trees on a circle, Discrete Math., 180, 301-313, 1998.
FORMULA
a(n) = 7/3*(n-1)*(n-2)*binomial(3*n, n)/(3*n-1)/(2*n+1)/(3*n-2) for n > 0; a(0)=1.
G.f.: g*(1+z-2*z*g), where g = 1+z*g^3 is the g.f. of the ternary numbers (A001764).
From Karol A. Penson, Mar 12 2018: (Start)
a(n+3) = 7*binomial(3*n+6, 2*n+6)/(2*n+7).
a(n+3) is the n-th moment of a signed function v(x) on (0,27/4), i.e., in Maple notation, a(n+3) = int(x^n*v(x) , x = 0..27/4), n = 0,1..., where v(x) = -sqrt(3)*x^(4/3)*(7*x^(1/3)*hypergeom([-5/6, -1/3, 8/3], [2/3, 4/3], 4*x/27))-3*hypergeom([-7/6, -2/3, 7/3], [1/3, 2/3], 4*x/27)))/(6*Pi). The function v(x) vanishes at x = 0 and x = 27/4. In addition it has one zero in the interval between x = 0 and x = 27/4. (End)
D-finite with recurrence 2*n*(2*n+1)*(n-3)*a(n) -3*(3*n-5)*(n-1)*(3*n-4)*a(n-1)=0. - R. J. Mathar, Jul 26 2022
EXAMPLE
a(2)=0 because in all the three noncrossing trees with 2 edges, namely, /_, /\ and _\, the root (=the top vertex) is incident with at least one border edge.
MAPLE
a:=n->7/3*(n-1)*(n-2)*binomial(3*n, n)/(3*n-1)/(2*n+1)/(3*n-2): 1, seq(a(n), n=1..25);
MATHEMATICA
a[0] = a[3] = 1; a[n_] := 7*Binomial[3n-3, 2n+1]/(n-3); Table[a[n], {n, 0, 22}] (* Jean-François Alcover, Jan 21 2013 *)
PROG
(PARI) a(n) = if (n==0, 1, 7/3*(n-1)*(n-2)*binomial(3*n, n)/(3*n-1)/(2*n+1)/(3*n-2)); \\ Michel Marcus, Oct 26 2015
(PARI) Vec((g->g*(1+x-2*x*g))(1+serreverse(x/(1+x)^3 + O(x^30)))) \\ Andrew Howroyd, Nov 17 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Emeric Deutsch, Jan 22 2005
STATUS
approved