G.f.: (1+x)/(1+x-2*x^2).

Terms

a(0) =1a(1) =0a(2) =2a(3) =-2a(4) =6a(5) =-10a(6) =22a(7) =-42a(8) =86a(9) =-170a(10) =342a(11) =-682a(12) =1366a(13) =-2730a(14) =5462a(15) =-10922a(16) =21846a(17) =-43690a(18) =87382a(19) =-174762a(20) =349526a(21) =-699050a(22) =1398102a(23) =-2796202a(24) =5592406a(25) =-11184810a(26) =22369622a(27) =-44739242a(28) =89478486a(29) =-178956970