a(n) = (2*2^n+7*(-1)^n)/3.

Terms

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