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 A216040 Number of permutations sortable using two parallel stacks. 4
 1, 1, 2, 6, 23, 103, 513, 2760, 15741, 93944, 581303, 3704045, 24180340, 161082639, 1091681427, 7508269793, 52302594344, 368422746908, 2620789110712, 18806093326963, 136000505625886, 990406677136685, 7258100272108212 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 LINKS Daniel Denton and Peter Doyle, Table of n, a(n) for n = 0..100 Daniel Denton, Methods of computing deque sortable permutations given complete and incomplete information, arXiv:1208.1532 [math.CO], 2012. Andrew Elvey-Price and Anthony J. Guttmann, Permutations sortable by deques and by two stacks in parallel, arxiv:1508.02273 [math.CO], 2015-2016. Andrew Elvey-Price and Anthony J. Guttmann, Permutations sortable by deques and by two stacks in parallel, European Journal of Combinatorics, 59 (2017), 71-95. EXAMPLE 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.) CROSSREFS Cf. A182216, A215252, A215257. Sequence in context: A301897 A022558 A004040 * A005802 A061552 A338752 Adjacent sequences:  A216037 A216038 A216039 * A216041 A216042 A216043 KEYWORD nonn AUTHOR Peter Doyle, Aug 30 2012 EXTENSIONS a(0)=1 added by N. J. A. Sloane, Sep 12 2012 STATUS approved

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Last modified January 25 12:00 EST 2021. Contains 340416 sequences. (Running on oeis4.)