a(n) = Sum_{k=0..floor(n/3)} C(n-k,2*k) * 2^k * (-2)^(n-3*k).

Terms

a(0) =1a(1) =-2a(2) =4a(3) =-6a(4) =4a(5) =16a(6) =-92a(7) =312a(8) =-848a(9) =1960a(10) =-3824a(11) =5760a(12) =-3824a(13) =-15392a(14) =88384a(15) =-299616a(16) =814144a(17) =-1881344a(18) =3669568a(19) =-5524608a(20) =3657472a(21) =14807680a(22) =-84909824a(23) =287723520a(24) =-781639424a(25) =1805843968a(26) =-3521371136a(27) =5298829824