a(n) = 2^n + 11.

Terms

a(0) =12a(1) =13a(2) =15a(3) =19a(4) =27a(5) =43a(6) =75a(7) =139a(8) =267a(9) =523a(10) =1035a(11) =2059a(12) =4107a(13) =8203a(14) =16395a(15) =32779a(16) =65547a(17) =131083a(18) =262155a(19) =524299a(20) =1048587a(21) =2097163a(22) =4194315a(23) =8388619a(24) =16777227a(25) =33554443a(26) =67108875a(27) =134217739a(28) =268435467a(29) =536870923