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

Terms

a(0) =1a(1) =6a(2) =37a(3) =232a(4) =1471a(5) =9402a(6) =60460a(7) =390656a(8) =2533987a(9) =16489546a(10) =107594213a(11) =703680424a(12) =4611412196a(13) =30273024984a(14) =199045392232