Largest prime <= 2^n.

Terms

a(0) =2a(1) =3a(2) =7a(3) =13a(4) =31a(5) =61a(6) =127a(7) =251a(8) =509a(9) =1021a(10) =2039a(11) =4093a(12) =8191a(13) =16381a(14) =32749a(15) =65521a(16) =131071a(17) =262139a(18) =524287a(19) =1048573a(20) =2097143a(21) =4194301a(22) =8388593a(23) =16777213a(24) =33554393a(25) =67108859a(26) =134217689a(27) =268435399a(28) =536870909a(29) =1073741789