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

Terms

a(0) =-1a(1) =2a(2) =-5a(3) =14a(4) =-41a(5) =122a(6) =-365a(7) =1094a(8) =-3281a(9) =9842a(10) =-29525a(11) =88574a(12) =-265721a(13) =797162a(14) =-2391485a(15) =7174454a(16) =-21523361a(17) =64570082a(18) =-193710245a(19) =581130734a(20) =-1743392201a(21) =5230176602a(22) =-15690529805a(23) =47071589414a(24) =-141214768241a(25) =423644304722