a(n)=(s(n)+5)/10, where s(n)=n-th base 10 palindrome that starts with 5.

Terms

a(0) =1a(1) =6a(2) =51a(3) =52a(4) =53a(5) =54a(6) =55a(7) =56a(8) =57a(9) =58a(10) =59a(11) =60a(12) =501a(13) =512a(14) =523a(15) =534a(16) =545a(17) =556a(18) =567a(19) =578a(20) =589a(21) =600a(22) =5001a(23) =5011a(24) =5021a(25) =5031a(26) =5041a(27) =5051a(28) =5061a(29) =5071