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

Terms

a(0) =1a(1) =3a(2) =-3a(3) =15a(4) =-39a(5) =123a(6) =-363a(7) =1095a(8) =-3279a(9) =9843a(10) =-29523a(11) =88575a(12) =-265719a(13) =797163a(14) =-2391483a(15) =7174455a(16) =-21523359a(17) =64570083a(18) =-193710243a(19) =581130735a(20) =-1743392199a(21) =5230176603a(22) =-15690529803a(23) =47071589415a(24) =-141214768239