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

Terms

a(0) =1a(1) =1a(2) =0a(3) =2a(4) =5a(5) =12a(6) =22a(7) =43a(8) =84a(9) =170a(10) =341a(11) =684a(12) =1366a(13) =2731a(14) =5460a(15) =10922a(16) =21845a(17) =43692a(18) =87382a(19) =174763a(20) =349524a(21) =699050a(22) =1398101a(23) =2796204a(24) =5592406a(25) =11184811a(26) =22369620a(27) =44739242a(28) =89478485a(29) =178956972