OFFSET
0,4
COMMENTS
a(n)=A182888(n,0).
REFERENCES
M. Bona and A. Knopfmacher, On the probability that certain compositions have the same number of parts, Ann. Comb., 14 (2010), 291-306.
E. Munarini, N. Zagaglia Salvi, On the rank polynomial of the lattice of order ideals of fences and crowns, Discrete Mathematics 259 (2002), 163-177.
FORMULA
G.f.: G(z) =1/( z+sqrt((1+z+z^2)(1-3z+z^2)) ).
a(n) ~ sqrt(7*sqrt(5)-15) * ((3 + sqrt(5))/2)^(n+2) / (sqrt(2*Pi) * n^(3/2)). - Vaclav Kotesovec, Mar 06 2016
Equivalently, a(n) ~ 5^(1/4) * phi^(2*n + 2) / (sqrt(Pi) * n^(3/2)), where phi = A001622 is the golden ratio. - Vaclav Kotesovec, Dec 06 2021
D-finite with recurrence n*a(n) +(-4*n+3)*a(n-1) +(n-3)*a(n-2) +(2*n-3)*a(n-3) +12*(n-3)*a(n-4) +(2*n-9)*a(n-5) +(n-3)*a(n-6) +(-4*n+21)*a(n-7) +(n-6)*a(n-8)=0. - R. J. Mathar, Jul 22 2022
EXAMPLE
a(3)=2. Indeed, denoting by h (H) the (1,0)-step of weight 1 (2), and u=(1,1), d=(1,-1), the five paths of weight 3 are ud, du, hH, Hh, and hhh; two of them, namely ud and du, have no h steps at level 0.
MAPLE
G:=1/(z+sqrt((1+z+z^2)*(1-3*z+z^2))): Gser:=series(G, z=0, 35): seq(coeff(Gser, z, n), n=0..32);
MATHEMATICA
CoefficientList[Series[1/(x+Sqrt[(1+x+x^2)(1-3x+x^2)]), {x, 0, 40}], x] (* Harvey P. Dale, Jun 16 2013 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Emeric Deutsch, Dec 11 2010
STATUS
approved