G.f.: 1/(1+x+x^3).

Terms

a(0) =1a(1) =-1a(2) =1a(3) =-2a(4) =3a(5) =-4a(6) =6a(7) =-9a(8) =13a(9) =-19a(10) =28a(11) =-41a(12) =60a(13) =-88a(14) =129a(15) =-189a(16) =277a(17) =-406a(18) =595a(19) =-872a(20) =1278a(21) =-1873a(22) =2745a(23) =-4023a(24) =5896a(25) =-8641a(26) =12664a(27) =-18560a(28) =27201a(29) =-39865