OFFSET
0,2
COMMENTS
A 3-Dyck path is a lattice path with steps U = (1, 3), d = (1, -1) that starts at (0,0), stays (weakly) above the x-axis, and ends at the x-axis.
LINKS
Andrei Asinowski, Benjamin Hackl, Sarah J. Selkirk, Down-step statistics in generalized Dyck paths, arXiv:2007.15562 [math.CO], 2020.
FORMULA
a(n) = binomial(4*(n+1)+1, n+1)/(4*(n+1)+1) - binomial(4*n+1, n)/(4*n+1).
a(n) = A062750(n+1, 3*n-1).
EXAMPLE
For n=2 the a(2)=18 is the total number of down-steps after the last up-step in UdddUddd, UddUdddd, UdUddddd, UUdddddd.
MAPLE
b:= proc(x, y) option remember; `if`(x=y, x,
`if`(y+3<x, b(x-1, y+3), 0)+`if`(y>0, b(x-1, y-1), 0))
end:
a:= n-> b(4*n, 0):
seq(a(n), n=0..21); # Alois P. Heinz, May 09 2020
# second Maple program:
a:= proc(n) option remember; `if`(n<2, 3*n, (8*(4*n-1)*
(2*n-1)*(4*n-3)*n*(229*n^2+303*n+98)*a(n-1))/
(3*(n-1)*(3*n+2)*(3*n+4)*(n+1)*(229*n^2-155*n+24)))
end:
seq(a(n), n=0..21); # Alois P. Heinz, May 09 2020
MATHEMATICA
nmax = 21;
A[_] = 0;
Do[A[x_] = 1 + x A[x]^4 + O[x]^(nmax + 2), nmax + 2];
CoefficientList[A[x], x] // Differences (* Jean-François Alcover, Aug 17 2020 *)
PROG
(PARI) a(n) = {binomial(4*(n+1)+1, n+1)/(4*(n+1)+1) - binomial(4*n+1, n)/(4*n+1)} \\ Andrew Howroyd, May 08 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Andrei Asinowski, May 08 2020
STATUS
approved