OFFSET
0,3
COMMENTS
FORMULA
G.f.=2(1+z+z^2)/[1-3z^2-z^4+sqrt((1+z^2-z^4)(1-3z^2-z^4))].
D-finite with recurrence (n+2)*a(n) +n*a(n-1) +(-n-6)*a(n-2) -2*n*a(n-3) +7*(-n+2)*a(n-4) +5*(-n+4)*a(n-5) +3*(-n+6)*a(n-6) +2*(n-8)*a(n-7) +(3*n-26)*a(n-8) +(n-8)*a(n-9) +(n-10)*a(n-10)=0. - R. J. Mathar, Oct 08 2016
EXAMPLE
a(4)=8 because we have hhhh, hHh, HH, hUDh, UDUD, UhhD, UHD and UUDD.
MAPLE
G:=(2*(1+z+z^2))/(1-3*z^2-z^4+sqrt((1+z^2-z^4)*(1-3*z^2-z^4))): Gser:=series(G, z=0, 35): seq(coeff(Gser, z, n), n=0..30);
CROSSREFS
KEYWORD
nonn
AUTHOR
Emeric Deutsch, Sep 05 2007
STATUS
approved