a(n) = (8*2^n-5*(-1)^n)/3.

Terms

a(0) =1a(1) =7a(2) =9a(3) =23a(4) =41a(5) =87a(6) =169a(7) =343a(8) =681a(9) =1367a(10) =2729a(11) =5463a(12) =10921a(13) =21847a(14) =43689a(15) =87383a(16) =174761a(17) =349527a(18) =699049a(19) =1398103a(20) =2796201a(21) =5592407a(22) =11184809a(23) =22369623a(24) =44739241a(25) =89478487a(26) =178956969a(27) =357913943a(28) =715827881a(29) =1431655767