OFFSET
0,3
FORMULA
a(n) = Sum_{k=0..floor(n/2)} binomial(4*n,k) * binomial(n-k,n-2*k).
The g.f. exp( Sum_{k>=1} a(k) * x^k/k ) has integer coefficients and equals (1/x) * Series_Reversion( x * (1+x)^3 / (1+x+x^2)^4 ). See A372383.
MATHEMATICA
a[n_]:=SeriesCoefficient[((1+x+x^2)^4/(1+x)^3)^n, {x, 0, n}]; Array[a, 25, 0] (* Stefano Spezia, Apr 30 2024 *)
PROG
(PARI) a(n, s=2, t=4, u=-3) = sum(k=0, n\s, binomial(t*n, k)*binomial((t+u)*n-k, n-s*k));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Apr 29 2024
STATUS
approved