a(n) = 3^n*(3^(n+1)+1)/2.

Terms

a(0) =2a(1) =15a(2) =126a(3) =1107a(4) =9882a(5) =88695a(6) =797526a(7) =7175547a(8) =64573362a(9) =581140575a(10) =5230206126a(11) =47071677987a(12) =423644570442