Semiprimes of the form 2^k + 1.

Terms

a(0) =9a(1) =33a(2) =65a(3) =129a(4) =2049a(5) =4097a(6) =8193a(7) =131073a(8) =524289a(9) =1048577a(10) =8388609a(11) =268435457a(12) =2147483649a(13) =4294967297