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 A248900 #27 Mar 06 2015 11:28:04 %S A248900 1,1,2,6,23,102,492,2492,13003,69172,372963,2031174,11148948,61588814, %T A248900 342068774,1908740089,10694374242,60137305751,339276548456, %U A248900 1919787554118,10892575241125,61957028188350,353224194632459,2018076850631391,11552829351121139,66259093178740462 %N A248900 Number of permutations of [n] avoiding patterns 2431 and 54321. %H A248900 Nathaniel Shar, <a href="/A248900/b248900.txt">Table of n, a(n) for n = 0..80</a> %H A248900 Nathaniel Shar, <a href="/A248900/a248900.txt">Maple code</a> %e A248900 a(4) = 23 because all permutations of length 4 except 2431 lie in this set. %Y A248900 Cf. A022558, A047889. %K A248900 nonn %O A248900 0,3 %A A248900 _Nathaniel Shar_, Mar 06 2015