Largest prime <= 5^n.

Terms

a(0) =5a(1) =23a(2) =113a(3) =619a(4) =3121a(5) =15619a(6) =78121a(7) =390581a(8) =1953109a(9) =9765619a(10) =48828113a(11) =244140613a(12) =1220703073a(13) =6103515623a(14) =30517578121a(15) =152587890563a(16) =762939453109