Dropping any binary digit gives a prime number.

Terms

a(0) =6a(1) =7a(2) =11a(3) =15a(4) =35a(5) =39a(6) =63a(7) =135a(8) =255a(9) =999a(10) =2175a(11) =8223a(12) =16383a(13) =57735a(14) =131075a(15) =131079a(16) =262143a(17) =524295a(18) =1048575a(19) =536870919a(20) =1073735679a(21) =2147483655a(22) =4294967295a(23) =17179770879