a(n) = (1 - (-3)^n)/4.

Terms

a(0) =0a(1) =1a(2) =-2a(3) =7a(4) =-20a(5) =61a(6) =-182a(7) =547a(8) =-1640a(9) =4921a(10) =-14762a(11) =44287a(12) =-132860a(13) =398581a(14) =-1195742a(15) =3587227a(16) =-10761680a(17) =32285041a(18) =-96855122a(19) =290565367a(20) =-871696100a(21) =2615088301a(22) =-7845264902a(23) =23535794707a(24) =-70607384120a(25) =211822152361a(26) =-635466457082