a(n) = a(n-1) + a(n-2) + 2*a(n-3) with a(0)=2, a(1)=1, a(2)=5.

Terms

a(0) =2a(1) =1a(2) =5a(3) =10a(4) =17a(5) =37a(6) =74a(7) =145a(8) =293a(9) =586a(10) =1169a(11) =2341a(12) =4682a(13) =9361a(14) =18725a(15) =37450a(16) =74897a(17) =149797a(18) =299594a(19) =599185a(20) =1198373a(21) =2396746a(22) =4793489a(23) =9586981a(24) =19173962a(25) =38347921a(26) =76695845a(27) =153391690a(28) =306783377a(29) =613566757