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 A369431 #26 Feb 25 2024 00:31:57
%S A369431 1,1,2,6,20,66,214,688,2206,7070,22660,72634,232830,746352,2392486,
%T A369431 7669286,24584436,78807122,252621702,809796400,2595858574
%N A369431 a(n) is the number of permutations of [n] which avoid the patterns 1234, 1324, 1342, and 2413.
%H A369431 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PermutationPattern.html">Permutation Pattern</a>
%H A369431 Wikipedia, <a href="https://en.wikipedia.org/wiki/Permutation_pattern">Permutation pattern</a>
%e A369431 For n = 4, the valid permutations are the 20 which are not elements of the set {1234,1324,1342,2413}, hence a(4) = 20.
%Y A369431 Cf. A033321 (avoiding 1234, 1324, 1342), A369626 (avoiding 1234, 1324, 2413), A053617 (avoiding 1234, 1324), A165530 (avoiding 1234 and 2413).
%K A369431 nonn,more
%O A369431 0,3
%A A369431 _Matt Slattery-Holmes_, Jan 23 2024
%E A369431 a(13)-a(20) from _Martin Ehrenstein_, Feb 24 2024