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 A217809 #11 Feb 17 2018 10:48:39 %S A217809 1,5,42,462,6070,90686,1493962,26584610,503663959 %N A217809 Number of alternating permutations on 2n letters that avoid a certain pattern of length 4 (see Lewis, 2012, Appendix, for precise definition). %H A217809 J. B. Lewis, <a href="https://dspace.mit.edu/handle/1721.1/73444">Pattern Avoidance for Alternating Permutations and Reading Words of Tableaux</a>, Ph. D. Dissertation, Department of Mathematics, MIT, 2012. %Y A217809 Cf. A181197, A217799-A217830. %K A217809 nonn,more %O A217809 1,2 %A A217809 _N. J. A. Sloane_, Oct 13 2012 %E A217809 a(7)-a(9) from _Lars Blomberg_, Feb 17 2018