A371615 G.f. satisfies A(x) = ( 1 + x / (1 - x*A(x)^3)^2 )^2.
1, 2, 5, 34, 222, 1622, 12559, 100904, 835322, 7070574, 60922335, 532566850, 4711614912, 42106192680, 379544358032, 3446755447528, 31504896429042, 289619348156494, 2675953520657839, 24836797229730316, 231461661673958896, 2165002179076830442
Offset: 0
Keywords
Programs
-
PARI
a(n, r=2, s=2, t=0, u=6) = r*sum(k=0, n, binomial(t*k+u*(n-k)+r, k)*binomial(n+(s-1)*k-1, n-k)/(t*k+u*(n-k)+r));
Formula
a(n) = Sum_{k=0..n} binomial(6*(n-k)+2,k) * binomial(n+k-1,n-k)/(3*(n-k)+1).