Largest prime <= 4^n.

Terms

a(0) =3a(1) =13a(2) =61a(3) =251a(4) =1021a(5) =4093a(6) =16381a(7) =65521a(8) =262139a(9) =1048573a(10) =4194301a(11) =16777213a(12) =67108859a(13) =268435399a(14) =1073741789a(15) =4294967291a(16) =17179869143a(17) =68719476731a(18) =274877906899