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

Terms

a(0) =1a(1) =5a(2) =-7a(3) =29a(4) =-79a(5) =245a(6) =-727a(7) =2189a(8) =-6559a(9) =19685a(10) =-59047a(11) =177149a(12) =-531439a(13) =1594325a(14) =-4782967a(15) =14348909a(16) =-43046719a(17) =129140165a(18) =-387420487a(19) =1162261469a(20) =-3486784399a(21) =10460353205a(22) =-31381059607a(23) =94143178829a(24) =-282429536479a(25) =847288609445