OFFSET
0,2
LINKS
FORMULA
G.f.: (1 + 2*x - x^2)/(1 - 2*x - x^2).
a(n) = 4 * A000129(n) for n >= 1.
EXAMPLE
There are 5 paths ending with x=2. Of these, UU has 3 unilevel points, UD has 1, L has none, DU has 1, and DD has 3.
MATHEMATICA
CoefficientList[Series[(1+2*x-x^2)/(1-2*x-x^2), {x, 0, 24}], x]
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Lou Shapiro, Mar 28 2024
STATUS
approved