A381860 G.f. A(x) satisfies A(x) = (1 + x)^3 * C(x*A(x)), where C(x) is the g.f. of A000108.
1, 4, 12, 55, 327, 2157, 15141, 110853, 836790, 6465309, 50876776, 406335099, 3285202335, 26835060422, 221128733649, 1835973630276, 15344202894457, 128983332603009, 1089803313492966, 9250137181234430, 78837133437062307, 674408139329393187, 5788618956395607745
Offset: 0
Keywords
Programs
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PARI
a(n) = sum(k=0, n, binomial(3*k+1, k)*binomial(3*k+3, n-k)/(3*k+1));
Formula
a(n) = Sum_{k=0..n} binomial(3*k+1,k) * binomial(3*k+3,n-k)/(3*k+1).
D-finite with recurrence -2*n*(2*n+1)*a(n) +3*(n^2+13*n-6)*a(n-1) +3*(69*n^2-221*n+150)*a(n-2) +2*(397*n^2-2431*n+3471)*a(n-3) +6*(225*n^2-1953*n+4079)*a(n-4) +9*(135*n^2-1503*n+4084)*a(n-5) +9*(63*n^2-855*n+2860)*a(n-6) +12*(3*n-22)*(3*n-26)*a(n-7)=0. - R. J. Mathar, Mar 10 2025