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

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =3a(5) =3a(6) =5a(7) =7a(8) =11a(9) =15a(10) =23a(11) =33a(12) =49a(13) =71a(14) =105a(15) =153a(16) =225a(17) =329a(18) =483a(19) =707a(20) =1037a(21) =1519a(22) =2227a(23) =3263a(24) =4783a(25) =7009a(26) =10273a(27) =15055a(28) =22065a(29) =32337