a(n) = 3^n - 2.

Terms

a(0) =1a(1) =7a(2) =25a(3) =79a(4) =241a(5) =727a(6) =2185a(7) =6559a(8) =19681a(9) =59047a(10) =177145a(11) =531439a(12) =1594321a(13) =4782967a(14) =14348905a(15) =43046719a(16) =129140161a(17) =387420487a(18) =1162261465a(19) =3486784399a(20) =10460353201a(21) =31381059607a(22) =94143178825a(23) =282429536479a(24) =847288609441