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

Terms

a(0) =1a(1) =1a(2) =-7a(3) =25a(4) =-79a(5) =241a(6) =-727a(7) =2185a(8) =-6559a(9) =19681a(10) =-59047a(11) =177145a(12) =-531439a(13) =1594321a(14) =-4782967a(15) =14348905a(16) =-43046719a(17) =129140161a(18) =-387420487a(19) =1162261465a(20) =-3486784399a(21) =10460353201a(22) =-31381059607a(23) =94143178825a(24) =-282429536479