OFFSET
0,2
COMMENTS
The Motzkin step set is U=(1,1), H=(1,0) and D=(1,-1). A meander is a path starting at (0,0) and never crossing the x-axis, i.e., staying at nonnegative altitude.
FORMULA
G.f.: -(1-2*t-t^2+t^3-sqrt(t^6+2*t^5-3*t^4-6*t^3-2*t^2+1))/(2*t*(1-2*t-t^2+t^3)).
EXAMPLE
a(3)=6 since we have 6 meanders of length 3, namely UUU, UUH, UHU, UHD, HUU and HUH.
CROSSREFS
KEYWORD
nonn,walk
AUTHOR
Valerie Roitner, Nov 29 2019
STATUS
approved