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

Terms

a(0) =0a(1) =0a(2) =2a(3) =1a(4) =2a(5) =8a(6) =20a(7) =45a(8) =108a(9) =264a(10) =638a(11) =1537a(12) =3710a(13) =8960a(14) =21632a(15) =52221a(16) =126072a(17) =304368a(18) =734810a(19) =1773985a(20) =4282778a(21) =10339544a(22) =24961868a(23) =60263277a(24) =145488420a(25) =351240120a(26) =847968662a(27) =2047177441a(28) =4942323542a(29) =11931824528