OFFSET
0,2
FORMULA
a(n) = binomial(n+3,3) - Sum_{k=0..n-4} a(k) * a(n-k-4).
G.f. A(x) satisfies: A(x) = 1/(1-x)^4 - x^4 * A(x)^2.
G.f.: 2 / ( (1-x)^2 * ((1-x)^2 + sqrt((1-x)^4 + 4*x^4)) ).
D-finite with recurrence (n+4)*a(n) +5*(-n-3)*a(n-1) +10*(n+2)*a(n-2) +10*(-n-1)*a(n-3) +(9*n-8)*a(n-4) +5*(-n+1)*a(n-5) =0. - R. J. Mathar, Jan 25 2023
PROG
(PARI) a(n) = sum(k=0, n\4, (-1)^k*binomial(n+3, 4*k+3)*binomial(2*k, k)/(k+1));
(PARI) my(N=40, x='x+O('x^N)); Vec(2/((1-x)^2*((1-x)^2+sqrt((1-x)^4+4*x^4))))
CROSSREFS
KEYWORD
sign
AUTHOR
Seiichi Manyama, Jan 23 2023
STATUS
approved