a(n) = 3^n + 3.

Terms

a(0) =4a(1) =6a(2) =12a(3) =30a(4) =84a(5) =246a(6) =732a(7) =2190a(8) =6564a(9) =19686a(10) =59052a(11) =177150a(12) =531444a(13) =1594326a(14) =4782972a(15) =14348910a(16) =43046724a(17) =129140166a(18) =387420492a(19) =1162261470a(20) =3486784404a(21) =10460353206a(22) =31381059612a(23) =94143178830a(24) =282429536484a(25) =847288609446