a(n) = 3*a(n-1) - 4*a(n-3), with a(0)=1, a(1)=2, a(2)=4, a(3)=9.

Terms

a(0) =1a(1) =2a(2) =4a(3) =9a(4) =19a(5) =41a(6) =87a(7) =185a(8) =391a(9) =825a(10) =1735a(11) =3641a(12) =7623a(13) =15929a(14) =33223a(15) =69177a(16) =143815a(17) =298553a(18) =618951a(19) =1281593a(20) =2650567a(21) =5475897a(22) =11301319a(23) =23301689a(24) =48001479a(25) =98799161a(26) =203190727a(27) =417566265a(28) =857502151a(29) =1759743545