a(n) = 4*a(n-2) - a(n-1), with a(0)=1, a(1)=-2.

Terms

a(0) =1a(1) =-2a(2) =6a(3) =-14a(4) =38a(5) =-94a(6) =246a(7) =-622a(8) =1606a(9) =-4094a(10) =10518a(11) =-26894a(12) =68966a(13) =-176542a(14) =452406a(15) =-1158574a(16) =2968198a(17) =-7602494a(18) =19475286a(19) =-49885262a(20) =127786406a(21) =-327327454a(22) =838473078a(23) =-2147782894a(24) =5501675206a(25) =-14092806782a(26) =36099507606a(27) =-92470734734