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

Terms

a(0) =1a(1) =2a(2) =4a(3) =8a(4) =17a(5) =36a(6) =76a(7) =160a(8) =337a(9) =710a(10) =1496a(11) =3152a(12) =6641a(13) =13992a(14) =29480a(15) =62112a(16) =130865a(17) =275722a(18) =580924a(19) =1223960a(20) =2578785a(21) =5433292a(22) =11447508a(23) =24118976a(24) =50816737a(25) =107066766a(26) =225581040a(27) =475281056a(28) =1001378849