a(n) = 2*a(n-1) + a(n-2), with a(0) = 1, a(1) = 2, a(2) = 4.

Terms

a(0) =1a(1) =2a(2) =4a(3) =10a(4) =24a(5) =58a(6) =140a(7) =338a(8) =816a(9) =1970a(10) =4756a(11) =11482a(12) =27720a(13) =66922a(14) =161564a(15) =390050a(16) =941664a(17) =2273378a(18) =5488420a(19) =13250218a(20) =31988856a(21) =77227930a(22) =186444716a(23) =450117362a(24) =1086679440a(25) =2623476242a(26) =6333631924a(27) =15290740090a(28) =36915112104a(29) =89120964298