a(n) = 4^(n+1) + 7.

Terms

a(0) =11a(1) =23a(2) =71a(3) =263a(4) =1031a(5) =4103a(6) =16391a(7) =65543a(8) =262151a(9) =1048583a(10) =4194311a(11) =16777223a(12) =67108871a(13) =268435463a(14) =1073741831a(15) =4294967303a(16) =17179869191a(17) =68719476743a(18) =274877906951