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

Terms

a(0) =1a(1) =6a(2) =38a(3) =245a(4) =1594a(5) =10429a(6) =68497a(7) =451166a(8) =2978230a(9) =19694622a(10) =130427529a(11) =864813846a(12) =5740250973a(13) =38135872725a(14) =253561166414