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 A093021 #15 Aug 17 2021 06:27:23 %S A093021 9,14,28,33,47,52,66,71,85,90,108,113,127,132,146,151,165,170,189,194, %T A093021 207,212,226,231,245,250,269,274,288,293,306,311,325,330,349,354,368, %U A093021 373,387,392,405,410,429,434,448,453,467,472,486,491,504,518,523,537 %N A093021 Indices of terms in A093019 with value 1. %C A093021 Integers which require a following 1 to have a valid Luhn mod 10 check digit. %H A093021 Reinhard Zumkeller, <a href="/A093021/b093021.txt">Table of n, a(n) for n = 1..10000</a> %H A093021 John Kilgo, <a href="https://web.archive.org/web/20040627030859/http://www.dotnetjohn.com/articles/articleid97.aspx">Using the Luhn Algorithm</a>, DotNetJohn.com. %H A093021 Webopedia, <a href="http://www.webopedia.com/TERM/L/Luhn_formula.html">Luhn formula</a> %H A093021 Wikipedia, <a href="http://en.wikipedia.org/wiki/Luhn_algorithm">Luhn algorithm</a> %H A093021 <a href="/index/De#decimal_expansion">Index entries for sequences related to decimal expansion of n</a> %F A093021 A093019(a(n)) = 1. - _Reinhard Zumkeller_, Nov 08 2014 %e A093021 14 is in the sequence because A093019(14)=1; 141 has a valid Luhn mod 10 check digit. %o A093021 (Haskell) %o A093021 a093021 n = a093021_list !! (n-1) %o A093021 a093021_list = filter ((== 1) . a093019) [0..] %o A093021 -- _Reinhard Zumkeller_, Nov 08 2014 %Y A093021 Cf. A093017-A093029. %K A093021 easy,nonn,base %O A093021 1,1 %A A093021 _Ray Chandler_, Apr 03 2004 %E A093021 Offset changed by _Reinhard Zumkeller_, Nov 08 2014