a(n) = (3^n - 3)/2.

Terms

a(0) =0a(1) =3a(2) =12a(3) =39a(4) =120a(5) =363a(6) =1092a(7) =3279a(8) =9840a(9) =29523a(10) =88572a(11) =265719a(12) =797160a(13) =2391483a(14) =7174452a(15) =21523359a(16) =64570080a(17) =193710243a(18) =581130732a(19) =1743392199a(20) =5230176600a(21) =15690529803a(22) =47071589412a(23) =141214768239