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 A008713 #19 Apr 22 2023 14:03:18 %S A008713 10,11,12,13,14,15,16,17,20,22,24,31,100,121,10000,1111111111111111 %N A008713 16 in base 16-n. %D A008713 J. J. Clessa, Micropuzzles, Pan Books, 1983, p. 31 (puzzle 51). %D A008713 GCHQ, The GCHQ Puzzle Book, Penguin, 2016. See pages 108 and 300. %H A008713 Albert Franck, <a href="http://www.paulcooijmans.com/others/puzfrank.html">Puzzles</a>, see item 4. %e A008713 16 = 121_3, so a(13) = 121. %o A008713 (PARI) a(n) = if (n <= 15, fromdigits(digits(16, 16-n), 10), 1111111111111111); \\ _Michel Marcus_, Aug 06 2017 %K A008713 fini,nonn,base,full,nice %O A008713 0,1 %A A008713 _N. J. A. Sloane_