a(n) = 19*2^n-1.

Terms

a(0) =18a(1) =37a(2) =75a(3) =151a(4) =303a(5) =607a(6) =1215a(7) =2431a(8) =4863a(9) =9727a(10) =19455a(11) =38911a(12) =77823a(13) =155647a(14) =311295a(15) =622591a(16) =1245183a(17) =2490367a(18) =4980735a(19) =9961471a(20) =19922943a(21) =39845887a(22) =79691775a(23) =159383551a(24) =318767103a(25) =637534207a(26) =1275068415a(27) =2550136831a(28) =5100273663a(29) =10200547327