a(n) = 0^n + 3^n - 1.

Terms

a(0) =1a(1) =2a(2) =8a(3) =26a(4) =80a(5) =242a(6) =728a(7) =2186a(8) =6560a(9) =19682a(10) =59048a(11) =177146a(12) =531440a(13) =1594322a(14) =4782968a(15) =14348906a(16) =43046720a(17) =129140162a(18) =387420488a(19) =1162261466a(20) =3486784400a(21) =10460353202a(22) =31381059608a(23) =94143178826a(24) =282429536480a(25) =847288609442