a(n) = 3*a(n-1) + 3*a(n-3) + a(n-4).

Terms

a(0) =1a(1) =3a(2) =9a(3) =30a(4) =100a(5) =330a(6) =1089a(7) =3597a(8) =11881a(9) =39240a(10) =129600a(11) =428040a(12) =1413721a(13) =4669203a(14) =15421329a(15) =50933190a(16) =168220900a(17) =555595890a(18) =1835008569a(19) =6060621597a(20) =20016873361a(21) =66111241680a(22) =218350598400a(23) =721163036880