a(n) = 2*(a(n-1) + a(n-3)) - a(n-4), with a(0) = 1, a(1) = 2, a(2) = 5 and a(3) = 12.

Terms

a(0) =1a(1) =2a(2) =5a(3) =12a(4) =27a(5) =62a(6) =143a(7) =328a(8) =753a(9) =1730a(10) =3973a(11) =9124a(12) =20955a(13) =48126a(14) =110527a(15) =253840a(16) =582977a(17) =1338882a(18) =3074917a(19) =7061948a(20) =16218683a(21) =37248318a(22) =85545615a(23) =196466648a(24) =451211249a(25) =1036265410a(26) =2379918501a(27) =5465792852a(28) =12552905275a(29) =28829382142