A217941(n)/2.

Terms

a(0) =1a(1) =3a(2) =7a(3) =16a(4) =33a(5) =70a(6) =141a(7) =287a(8) =578a(9) =1163a(10) =2327a(11) =4674a(12) =9349a(13) =18718a(14) =37452a(15) =74948a(16) =149897a(17) =299890a(18) =599781a(19) =1199724a(20) =2399498a(21) =4799278a(22) =9598557a(23) =19197792a(24) =38395600a(25) =76792313a(26) =153584811a(27) =307171904a(28) =614343809a(29) =1228692446