a(n) = (7 - 4*(-2)^n)/3.

Terms

a(0) =1a(1) =5a(2) =-3a(3) =13a(4) =-19a(5) =45a(6) =-83a(7) =173a(8) =-339a(9) =685a(10) =-1363a(11) =2733a(12) =-5459a(13) =10925a(14) =-21843a(15) =43693a(16) =-87379a(17) =174765a(18) =-349523a(19) =699053a(20) =-1398099a(21) =2796205a(22) =-5592403a(23) =11184813a(24) =-22369619a(25) =44739245a(26) =-89478483a(27) =178956973a(28) =-357913939a(29) =715827885