a(n)=(s(n)+3)/9, where s(n)=n-th base 9 palindrome that starts with 6.

Terms

a(0) =1a(1) =7a(2) =55a(3) =56a(4) =57a(5) =58a(6) =59a(7) =60a(8) =61a(9) =62a(10) =63a(11) =487a(12) =497a(13) =507a(14) =517a(15) =527a(16) =537a(17) =547a(18) =557a(19) =567a(20) =4375a(21) =4384a(22) =4393a(23) =4402a(24) =4411a(25) =4420a(26) =4429a(27) =4438a(28) =4447a(29) =4457