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 A216040 #28 May 31 2020 21:57:50 %S A216040 1,1,2,6,23,103,513,2760,15741,93944,581303,3704045,24180340, %T A216040 161082639,1091681427,7508269793,52302594344,368422746908, %U A216040 2620789110712,18806093326963,136000505625886,990406677136685,7258100272108212 %N A216040 Number of permutations sortable using two parallel stacks. %H A216040 Daniel Denton and Peter Doyle, <a href="/A216040/b216040.txt">Table of n, a(n) for n = 0..100</a> %H A216040 Daniel Denton, <a href="http://arxiv.org/abs/1208.1532">Methods of computing deque sortable permutations given complete and incomplete information</a>, arXiv:1208.1532 [math.CO], 2012. %H A216040 Andrew Elvey-Price and Anthony J. Guttmann, <a href="http://arxiv.org/abs/1508.02273">Permutations sortable by deques and by two stacks in parallel</a>, arxiv:1508.02273 [math.CO], 2015-2016. %H A216040 Andrew Elvey-Price and Anthony J. Guttmann, <a href="https://doi.org/10.1016/j.ejc.2016.08.002">Permutations sortable by deques and by two stacks in parallel</a>, European Journal of Combinatorics, 59 (2017), 71-95. %e A216040 Up to n = 4, the only permutation that can't be sorted is 2341. This fails because after moving 2 to one stack, you must move 3 to the other stack, and now the 4 will block either the 2 or the 3. (If you use a double-ended queue instead of two stacks, then this permutation becomes sortable; cf. A182216.) %Y A216040 Cf. A182216, A215252, A215257. %K A216040 nonn %O A216040 0,3 %A A216040 _Peter Doyle_, Aug 30 2012 %E A216040 a(0)=1 added by _N. J. A. Sloane_, Sep 12 2012