a(n) = 3*a(n-1) - a(n-3) + 3*a(n-4), with initial values 0,0,0,1.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =3a(5) =9a(6) =26a(7) =78a(8) =234a(9) =703a(10) =2109a(11) =6327a(12) =18980a(13) =56940a(14) =170820a(15) =512461a(16) =1537383a(17) =4612149a(18) =13836446a(19) =41509338a(20) =124528014a(21) =373584043a(22) =1120752129a(23) =3362256387a(24) =10086769160a(25) =30260307480a(26) =90780922440a(27) =272342767321a(28) =817028301963