a(n) = 6*4^n - 1.

Terms

a(0) =5a(1) =23a(2) =95a(3) =383a(4) =1535a(5) =6143a(6) =24575a(7) =98303a(8) =393215a(9) =1572863a(10) =6291455a(11) =25165823a(12) =100663295a(13) =402653183a(14) =1610612735a(15) =6442450943a(16) =25769803775a(17) =103079215103a(18) =412316860415