a(0) = a(1) = a(2) = a(3) = 1; thereafter a(n) = a(n-1) + a(n-2) + a(n-4).

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =3a(5) =5a(6) =9a(7) =15a(8) =27a(9) =47a(10) =83a(11) =145a(12) =255a(13) =447a(14) =785a(15) =1377a(16) =2417a(17) =4241a(18) =7443a(19) =13061a(20) =22921a(21) =40223a(22) =70587a(23) =123871a(24) =217379a(25) =381473a(26) =669439a(27) =1174783a(28) =2061601a(29) =3617857