a(n) = Sum_{k=0..n} (-1)^k * binomial(3*n+k,n-k).

Terms

a(0) =1a(1) =2a(2) =9a(3) =49a(4) =286a(5) =1728a(6) =10660a(7) =66691a(8) =421497a(9) =2684605a(10) =17203264a(11) =110784699a(12) =716332655a(13) =4647649736a(14) =30242407617a(15) =197283062901