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 A071819 #9 Dec 29 2022 06:46:25
%S A071819 1,1,1,2,3,23,5,144,98,658
%N A071819 a(n) is the difference between the two smallest positive integers that you cannot obtain using the integers {1,2,...,n} at most once and the four operators +, -, *, /.
%H A071819 Gilles Bannay, <a href="https://web.archive.org/web/20061201125224/http://gilles.bannay.free.fr/jeux_us.html">Countdown Problem</a>
%H A071819 <a href="/index/Fo#4x4">Index entries for similar sequences</a>
%e A071819 a(4)=2 because 29 and 31 are the smallest numbers that you cannot obtain using {1,2,3,4} and the four operations and 31-29 = 2.
%Y A071819 Cf. A060315.
%K A071819 hard,more,nonn
%O A071819 1,4
%A A071819 Koksal Karakus (karakusk(AT)hotmail.com), Jun 07 2002