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 A319253 #6 Sep 16 2018 16:41:52
%S A319253 1,3,31,1186
%N A319253 Number of doubly sorted permutations of [2n].
%C A319253 A permutation pi is called doubly sorted if |s^{-1}(pi)| = 2, where s is West's stack-sorting map. There are no doubly sorted permutations of odd length.
%H A319253 C. Defant, M. Engen, and J. A. Miller, <a href="https://arxiv.org/abs/1809.01340"> Stack-sorting, set partitions, and Lassalle's sequence</a>, arXiv:1809.01340 [math.CO], 2018.
%e A319253 The a(2) = 3 doubly sorted permutations of [4] are 2314, 1324, and 3124. Specifically, letting s denote West's stack-sorting map, we have s^{-1}(2314) = {2341, 3241}, s^{-1}(1324) = {1342, 3142}, and s^{-1}(3124) = {3412, 3421}.
%Y A319253 Cf. A180874, A317551.
%K A319253 nonn,more
%O A319253 1,2
%A A319253 _Colin Defant_, Sep 15 2018