a(n)=2a(n-1)+a(n-2)-4a(n-4).

Terms

a(0) =1a(1) =1a(2) =3a(3) =7a(4) =13a(5) =29a(6) =59a(7) =119a(8) =245a(9) =493a(10) =995a(11) =2007a(12) =4029a(13) =8093a(14) =16235a(15) =32535a(16) =65189a(17) =130541a(18) =261331a(19) =523063a(20) =1046701a(21) =2094301a(22) =4189979a(23) =8382007a(24) =16767189a(25) =33539181a(26) =67085635a(27) =134182423a(28) =268381725a(29) =536789149