a(n) = 10*3^n - 1.

Terms

a(0) =9a(1) =29a(2) =89a(3) =269a(4) =809a(5) =2429a(6) =7289a(7) =21869a(8) =65609a(9) =196829a(10) =590489a(11) =1771469a(12) =5314409a(13) =15943229a(14) =47829689a(15) =143489069a(16) =430467209a(17) =1291401629a(18) =3874204889a(19) =11622614669a(20) =34867844009a(21) =104603532029a(22) =313810596089a(23) =941431788269