a(n) = 2^n + 2*n + 1.

Terms

a(0) =2a(1) =5a(2) =9a(3) =15a(4) =25a(5) =43a(6) =77a(7) =143a(8) =273a(9) =531a(10) =1045a(11) =2071a(12) =4121a(13) =8219a(14) =16413a(15) =32799a(16) =65569a(17) =131107a(18) =262181a(19) =524327a(20) =1048617a(21) =2097195a(22) =4194349a(23) =8388655a(24) =16777265a(25) =33554483a(26) =67108917a(27) =134217783a(28) =268435513a(29) =536870971