Semiprime nearest to 2^n. (In case of a tie, choose the smaller).

Terms

a(0) =4a(1) =4a(2) =4a(3) =9a(4) =15a(5) =33a(6) =65a(7) =129a(8) =254a(9) =511a(10) =1027a(11) =2047a(12) =4097a(13) =8193a(14) =16382a(15) =32765a(16) =65531a(17) =131073a(18) =262142a(19) =524289a(20) =1048577a(21) =2097149a(22) =4194311a(23) =8388607a(24) =16777219a(25) =33554429a(26) =67108867a(27) =134217731a(28) =268435457a(29) =536870918