a(1)=1, then a(n)=2*a(n-1) if n is prime, a(n)=2*a(n-1)+1 if n not prime.

Terms

a(0) =1a(1) =2a(2) =4a(3) =9a(4) =18a(5) =37a(6) =74a(7) =149a(8) =299a(9) =599a(10) =1198a(11) =2397a(12) =4794a(13) =9589a(14) =19179a(15) =38359a(16) =76718a(17) =153437a(18) =306874a(19) =613749a(20) =1227499a(21) =2454999a(22) =4909998a(23) =9819997a(24) =19639995a(25) =39279991a(26) =78559983a(27) =157119967a(28) =314239934a(29) =628479869