a(n) = 1 + a(n-1) + a(n-2) + a(n-3) if n>=4; a(1) = a(2) = a(3) = 1.

Terms

a(0) =1a(1) =1a(2) =1a(3) =4a(4) =7a(5) =13a(6) =25a(7) =46a(8) =85a(9) =157a(10) =289a(11) =532a(12) =979a(13) =1801a(14) =3313a(15) =6094a(16) =11209a(17) =20617a(18) =37921a(19) =69748a(20) =128287a(21) =235957a(22) =433993a(23) =798238a(24) =1468189a(25) =2700421a(26) =4966849a(27) =9135460a(28) =16802731a(29) =30905041