a(n) = Sum_{k=0..n} binomial(3*n+3*k+2,n-k).

Terms

a(0) =1a(1) =6a(2) =40a(3) =274a(4) =1895a(5) =13159a(6) =91545a(7) =637392a(8) =4439348a(9) =30921149a(10) =215353449a(11) =1499589488a(12) =10439943466a(13) =72664036557a(14) =505629773252