OFFSET
0,2
FORMULA
a(n) = n+1 - Sum_{k=0..n-2} a(k) * a(n-k-2).
G.f. A(x) satisfies: A(x) = 1/(1-x)^2 - x^2 * A(x)^2.
G.f.: 2 / ( (1-x) * (1-x + sqrt((1-x)^2 + 4*x^2)) ).
D-finite with recurrence (n+2)*a(n) +3*(-n-1)*a(n-1) +(7*n-4)*a(n-2) +5*(-n+1)*a(n-3)=0. - R. J. Mathar, Jan 25 2023
PROG
(PARI) a(n) = sum(k=0, n\2, (-1)^k*binomial(n+1, 2*k+1)*binomial(2*k, k)/(k+1));
(PARI) my(N=40, x='x+O('x^N)); Vec(2/((1-x)*(1-x+sqrt((1-x)^2+4*x^2))))
CROSSREFS
KEYWORD
sign
AUTHOR
Seiichi Manyama, Jan 23 2023
STATUS
approved