a(n) = 2^n + 7.

Terms

a(0) =8a(1) =9a(2) =11a(3) =15a(4) =23a(5) =39a(6) =71a(7) =135a(8) =263a(9) =519a(10) =1031a(11) =2055a(12) =4103a(13) =8199a(14) =16391a(15) =32775a(16) =65543a(17) =131079a(18) =262151a(19) =524295a(20) =1048583a(21) =2097159a(22) =4194311a(23) =8388615a(24) =16777223a(25) =33554439a(26) =67108871a(27) =134217735a(28) =268435463a(29) =536870919