Largest prime <= 8^n.

Terms

a(0) =7a(1) =61a(2) =509a(3) =4093a(4) =32749a(5) =262139a(6) =2097143a(7) =16777213a(8) =134217689a(9) =1073741789a(10) =8589934583a(11) =68719476731a(12) =549755813881