a(n) = 3*a(n-2) - a(n-1) for n > 1, a(0) = 0, a(1) = 1.

Terms

a(0) =0a(1) =1a(2) =-1a(3) =4a(4) =-7a(5) =19a(6) =-40a(7) =97a(8) =-217a(9) =508a(10) =-1159a(11) =2683a(12) =-6160a(13) =14209a(14) =-32689a(15) =75316a(16) =-173383a(17) =399331a(18) =-919480a(19) =2117473a(20) =-4875913a(21) =11228332a(22) =-25856071a(23) =59541067a(24) =-137109280a(25) =315732481a(26) =-727060321a(27) =1674257764a(28) =-3855438727a(29) =8878212019