a(n) = 2*a(n-1) + 2*a(n-3) + a(n-4), with a(0)=1, a(1)=2, a(3)=4, a(4)=10.

Terms

a(0) =1a(1) =2a(2) =4a(3) =10a(4) =25a(5) =60a(6) =144a(7) =348a(8) =841a(9) =2030a(10) =4900a(11) =11830a(12) =28561a(13) =68952a(14) =166464a(15) =401880a(16) =970225a(17) =2342330a(18) =5654884a(19) =13652098a(20) =32959081a(21) =79570260a(22) =192099600a(23) =463769460a(24) =1119638521a(25) =2703046502a(26) =6525731524a(27) =15754509550a(28) =38034750625a(29) =91824010800