a(n) = 3^n - 4.

Terms

a(0) =-3a(1) =-1a(2) =5a(3) =23a(4) =77a(5) =239a(6) =725a(7) =2183a(8) =6557a(9) =19679a(10) =59045a(11) =177143a(12) =531437a(13) =1594319a(14) =4782965a(15) =14348903a(16) =43046717a(17) =129140159a(18) =387420485a(19) =1162261463a(20) =3486784397a(21) =10460353199a(22) =31381059605a(23) =94143178823a(24) =282429536477a(25) =847288609439