a(1) = a(2) = 1, a(3) = 9; for n > 3, a(n) = 3*a(n-1) - 3*a(n-2) + 9*a(n-3).

Terms

a(0) =1a(1) =1a(2) =9a(3) =33a(4) =81a(5) =225a(6) =729a(7) =2241a(8) =6561a(9) =19521a(10) =59049a(11) =177633a(12) =531441a(13) =1592865a(14) =4782969a(15) =14353281a(16) =43046721a(17) =129127041a(18) =387420489a(19) =1162300833a(20) =3486784401a(21) =10460235105a(22) =31381059609a(23) =94143533121a(24) =282429536481a(25) =847287546561