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 A035239 #18 May 11 2021 05:25:50 %S A035239 1,12,123,1234,12345,123456,1234567,12345678,123456789,1023456789, %T A035239 11023456789,101123456789,1011213456789,10112131456789, %U A035239 101121314156789,1011213141516789,10112131415161789,101121314151617189 %N A035239 Smallest number that contains the numbers from 1 to n as substrings. %H A035239 D. Rusin, <a href="https://web.archive.org/web/20120721032225/http://www.math.niu.edu:80/~rusin/known-math/95/1000digits">Solution for n=1000</a> %H A035239 D. Rusin, <a href="/A035239/a035239.txt">Solution for n=1000</a> [Cached copy] %Y A035239 Cf. A054260, A054261, A056744. %K A035239 easy,nonn,base %O A035239 1,2 %A A035239 _Erich Friedman_ %E A035239 Keyword base added by _Reinhard Zumkeller_, May 16 2010