a(n) = 2*(a(n-1) + a(n-2) + a(n-3)) - a(n-4) for n >= 4, with initial terms 0,0,0,1.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =2a(5) =6a(6) =18a(7) =51a(8) =148a(9) =428a(10) =1236a(11) =3573a(12) =10326a(13) =29842a(14) =86246a(15) =249255a(16) =720360a(17) =2081880a(18) =6016744a(19) =17388713a(20) =50254314a(21) =145237662a(22) =419744634a(23) =1213084507a(24) =3505879292a(25) =10132179204a(26) =29282541372a(27) =84628115229a(28) =244579792318a(29) =706848718634