This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A057139 #8 Jan 28 2020 16:13:12
%S A057139 1,121,12321,1234321,123454321,12345654321,1234567654321,
%T A057139 123456787654321,12345678987654321,1234567890987654321,
%U A057139 123456789010987654321,12345678901210987654321,1234567890123210987654321,123456789012343210987654321,12345678901234543210987654321
%N A057139 Odd number of digits palindrome based on sequential digits.
%H A057139 Andrew Howroyd, <a href="/A057139/b057139.txt">Table of n, a(n) for n = 1..100</a>
%F A057139 a(n) = 10^n*A057137(n-1) + A057138(n) = 10^(n-1)*A057137(n) + A057138(n-1).
%t A057139 Array[FromDigits@ Join[#, Reverse@ Most@ #] &@ Mod[Range[#], 10] &, 15] (* _Michael De Vlieger_, Jan 28 2020 *)
%o A057139 (PARI) a(n)={fromdigits(vector(2*n-1, i, if(i<=n, i, 2*n-i)%10))} \\ _Andrew Howroyd_, Jan 27 2020
%Y A057139 Alternative progression for n >= 10 compared with A002477.
%Y A057139 Cf. A057137, A057138.
%K A057139 base,easy,nonn
%O A057139 1,2
%A A057139 _Henry Bottomley_, Aug 12 2000
%E A057139 Terms a(13) and beyond from _Andrew Howroyd_, Jan 27 2020