OFFSET
0,5
COMMENTS
Column 0 of A121481.
LINKS
E. Barcucci, A. Del Lungo, S. Fezzi and R. Pinzani, Nondecreasing Dyck paths and q-Fibonacci numbers, Discrete Math., 170, 1997, 211-217.
Index entries for linear recurrences with constant coefficients, signature (1,4,-2,-4,0,1).
FORMULA
G.f.: (1-z-z^2)(1-2z^2)/(1-z-4z^2+2z^3+4z^4-z^6).
EXAMPLE
a(4)=3 because we have UUDDUUDD, UUDUDUDD and UUUUDDDD, where U=(1,1) and D=(1,-1).
MAPLE
G:=(1-z-z^2)*(1-2*z^2)/(1-z-4*z^2+2*z^3+4*z^4-z^6): Gser:=series(G, z=0, 40): seq(coeff(Gser, z, n), n=0..37);
CROSSREFS
KEYWORD
nonn
AUTHOR
Emeric Deutsch, Aug 02 2006
STATUS
approved