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

Terms

a(0) =0a(1) =4a(2) =8a(3) =28a(4) =80a(5) =244a(6) =728a(7) =2188a(8) =6560a(9) =19684a(10) =59048a(11) =177148a(12) =531440a(13) =1594324a(14) =4782968a(15) =14348908a(16) =43046720a(17) =129140164a(18) =387420488a(19) =1162261468a(20) =3486784400a(21) =10460353204a(22) =31381059608a(23) =94143178828a(24) =282429536480a(25) =847288609444