a(n) = 9*(3^n - 1)/2.

Terms

a(0) =0a(1) =9a(2) =36a(3) =117a(4) =360a(5) =1089a(6) =3276a(7) =9837a(8) =29520a(9) =88569a(10) =265716a(11) =797157a(12) =2391480a(13) =7174449a(14) =21523356a(15) =64570077a(16) =193710240a(17) =581130729a(18) =1743392196a(19) =5230176597a(20) =15690529800a(21) =47071589409a(22) =141214768236a(23) =423644304717