a(1) = 1, a(2) = 2, a(3) = 5; thereafter a(n) = 2 * Sum_{k=1..n-1} a(k).

Terms

a(0) =1a(1) =2a(2) =5a(3) =16a(4) =48a(5) =144a(6) =432a(7) =1296a(8) =3888a(9) =11664a(10) =34992a(11) =104976a(12) =314928a(13) =944784a(14) =2834352a(15) =8503056a(16) =25509168a(17) =76527504a(18) =229582512a(19) =688747536a(20) =2066242608a(21) =6198727824a(22) =18596183472a(23) =55788550416a(24) =167365651248a(25) =502096953744