a(n) = 4*5^n + 1.

Terms

a(0) =5a(1) =21a(2) =101a(3) =501a(4) =2501a(5) =12501a(6) =62501a(7) =312501a(8) =1562501a(9) =7812501a(10) =39062501a(11) =195312501a(12) =976562501a(13) =4882812501a(14) =24414062501a(15) =122070312501a(16) =610351562501