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

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =4a(5) =6a(6) =9a(7) =15a(8) =25a(9) =40a(10) =64a(11) =104a(12) =169a(13) =273a(14) =441a(15) =714a(16) =1156a(17) =1870a(18) =3025a(19) =4895a(20) =7921a(21) =12816a(22) =20736a(23) =33552a(24) =54289a(25) =87841a(26) =142129a(27) =229970a(28) =372100a(29) =602070