a(n) = (7-(-5)^n)/6.

Terms

a(0) =1a(1) =2a(2) =-3a(3) =22a(4) =-103a(5) =522a(6) =-2603a(7) =13022a(8) =-65103a(9) =325522a(10) =-1627603a(11) =8138022a(12) =-40690103a(13) =203450522a(14) =-1017252603a(15) =5086263022a(16) =-25431315103a(17) =127156575522a(18) =-635782877603