a(n) = (27*3^n - 63)/2.

Terms

a(0) =9a(1) =90a(2) =333a(3) =1062a(4) =3249a(5) =9810a(6) =29493a(7) =88542a(8) =265689a(9) =797130a(10) =2391453a(11) =7174422a(12) =21523329a(13) =64570050a(14) =193710213a(15) =581130702a(16) =1743392169a(17) =5230176570a(18) =15690529773a(19) =47071589382a(20) =141214768209a(21) =423644304690