a(n) = (-1)^n * (3^n - 1)/2.

Terms

a(0) =0a(1) =-1a(2) =4a(3) =-13a(4) =40a(5) =-121a(6) =364a(7) =-1093a(8) =3280a(9) =-9841a(10) =29524a(11) =-88573a(12) =265720a(13) =-797161a(14) =2391484a(15) =-7174453a(16) =21523360a(17) =-64570081a(18) =193710244a(19) =-581130733a(20) =1743392200a(21) =-5230176601a(22) =15690529804a(23) =-47071589413a(24) =141214768240a(25) =-423644304721