a(n) = 3^(-1-floor(n/3))*A215829(n).

Terms

a(0) =1a(1) =-1a(2) =9a(3) =-11a(4) =59a(5) =-267a(6) =433a(7) =-2041a(8) =9753a(9) =-15443a(10) =73547a(11) =-349875a(12) =555073a(13) =-2641297a(14) =12569673a(15) =-19938491a(16) =94883195a(17) =-451526331a(18) =716237425a(19) =-3408408073a(20) =16219834713a(21) =-25728821507a(22) =122437560587a(23) =-582652240611a(24) =924236100865