a(n) = (s(n) + 1)/5, where s(n) = n-th base-5 palindrome that starts with 4.

Terms

a(0) =1a(1) =5a(2) =21a(3) =22a(4) =23a(5) =24a(6) =25a(7) =101a(8) =107a(9) =113a(10) =119a(11) =125a(12) =501a(13) =506a(14) =511a(15) =516a(16) =521a(17) =527a(18) =532a(19) =537a(20) =542a(21) =547a(22) =553a(23) =558a(24) =563a(25) =568a(26) =573a(27) =579a(28) =584a(29) =589