a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3) + a(n-4) with a(1) = 1, a(2) = 2, a(3) = 4, and a(4) = 7.

Terms

a(0) =1a(1) =2a(2) =4a(3) =7a(4) =12a(5) =21a(6) =38a(7) =70a(8) =129a(9) =236a(10) =429a(11) =778a(12) =1412a(13) =2567a(14) =4672a(15) =8505a(16) =15478a(17) =28158a(18) =51217a(19) =93160a(20) =169465a(21) =308290a(22) =560852a(23) =1020311a(24) =1856132a(25) =3376605a(26) =6142582a(27) =11174374a(28) =20328113a(29) =36980404