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

Terms

a(0) =2a(1) =3a(2) =4a(3) =9a(4) =18a(5) =39a(6) =80a(7) =165a(8) =334a(9) =675a(10) =1356a(11) =2721a(12) =5450a(13) =10911a(14) =21832a(15) =43677a(16) =87366a(17) =174747a(18) =349508a(19) =699033a(20) =1398082a(21) =2796183a(22) =5592384a(23) =11184789a(24) =22369598a(25) =44739219a(26) =89478460a(27) =178956945a(28) =357913914a(29) =715827855