Composites of the form 2^n-1 or 2^n+1 that are non-multiples of 3.

Terms

a(0) =65a(1) =511a(2) =1025a(3) =2047a(4) =4097a(5) =16385a(6) =32767a(7) =262145a(8) =1048577a(9) =2097151a(10) =4194305a(11) =8388607a(12) =16777217a(13) =33554431a(14) =67108865a(15) =134217727a(16) =268435457a(17) =536870911a(18) =1073741825a(19) =4294967297a(20) =8589934591a(21) =17179869185a(22) =34359738367a(23) =68719476737a(24) =137438953471a(25) =274877906945