OFFSET
0,2
FORMULA
a(n) = [x^n] ( (1+x)^2/(1-x)^2 )^n.
The g.f. exp( Sum_{k>=1} a(k) * x^k/k ) has integer coefficients and equals (1/x) * Series_Reversion( x*(1-x)^2/(1+x)^2 ).
a(n) = 2 * A103885(n) for n >= 1. - Peter Bala, Sep 16 2024
PROG
(PARI) a(n) = sum(k=0, n, binomial(2*n, k)*binomial(3*n-k-1, n-k));
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Seiichi Manyama, Feb 10 2024
STATUS
approved