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

Terms

a(0) =1a(1) =1a(2) =4a(3) =5a(4) =6a(5) =10a(6) =15a(7) =21a(8) =31a(9) =46a(10) =67a(11) =98a(12) =144a(13) =211a(14) =309a(15) =453a(16) =664a(17) =973a(18) =1426a(19) =2090a(20) =3063a(21) =4489a(22) =6579a(23) =9642a(24) =14131a(25) =20710a(26) =30352a(27) =44483a(28) =65193a(29) =95545