a(0) = 1, a(1) = 2; for n > 0, a(2n) = 3a(2n-1), a(2n+1) = 3a(2n) - 2a(n-1).

Terms

a(0) =1a(1) =2a(2) =6a(3) =16a(4) =48a(5) =140a(6) =420a(7) =1248a(8) =3744a(9) =11200a(10) =33600a(11) =100704a(12) =302112a(13) =906056a(14) =2718168a(15) =8153664a(16) =24460992a(17) =73380480a(18) =220141440a(19) =660416832a(20) =1981250496a(21) =5943729088a(22) =17831187264a(23) =53493494592a(24) =160480483776a(25) =481441249920