OFFSET
2,2
COMMENTS
This sequence is related to paired pattern P_2 in Section 3.2 in Pan and Remmel's link.
By symmetry, a(n) is also the number of North-East lattice paths from (0,0) to (n,n) that bounce off the diagonal y = x to the left exactly once.
LINKS
John Tyler Rascoe, Table of n, a(n) for n = 2..100
Ran Pan and Jeffrey B. Remmel, Paired patterns in lattice paths, arXiv:1601.07988 [math.CO], 2016.
FORMULA
G.f.: (-1 + f(x) + 2*x)^2/(1 - f(x) + (-5 + f(x))*x)^2, where f(x) = sqrt(1 - 4*x).
MATHEMATICA
(1 - 4x + x^2 - 2x^3 - Sqrt[1-4x](1 - 2x - 3x^2))/(2(-1 + x(4+x))^2) + O[x]^31 // CoefficientList[#, x]& // Drop[#, 2]& (* Jean-François Alcover, Dec 15 2018 *)
PROG
(PARI)
A_x(N) = {my(x='x+O('x^N), f=sqrt(1-4*x)); Vec((-1+f+2*x)^2/(1-f+(-5+f)*x)^2)}
A_x(31) \\ John Tyler Rascoe, Aug 26 2024
CROSSREFS
KEYWORD
nonn
AUTHOR
Ran Pan, Feb 04 2016
STATUS
approved