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A164870
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The number of permutations of length n that can be sorted by 2 pop stacks in parallel.
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2
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1, 2, 6, 22, 84, 320, 1212, 4576, 17256, 65048, 245184, 924160, 3483408, 13129952, 49490592, 186544480, 703140672, 2650342784, 9989916864, 37654917376, 141932392320, 534984681344, 2016513669120, 7600829555200, 28649748728064, 107989278831104, 407043163037184
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OFFSET
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1,2
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LINKS
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Colin Barker, Table of n, a(n) for n = 1..1000
Anders Claesson, Mark Dukes and Martina Kubitzke, Partition and composition matrices, arXiv:1006.1312 [math.CO], 2010-2011.
R. Smith and V. Vatter, The enumeration of permutations sortable by pop stacks in parallel, Information Processing Letters, Volume 109, Issue 12, 31 May 2009, Pages 626-629.
Index entries for linear recurrences with constant coefficients, signature (6,-10,6)
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FORMULA
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G.f.: -x*(2*x-1)^2 / ( -1+6*x-10*x^2+6*x^3 ).
a(n) = 6*a(n-1) - 10*a(n-2) + 6*a(n-3) for n>3. - Colin Barker, Oct 31 2017
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PROG
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(PARI) Vec(x*(1 - 2*x)^2 / (1 - 6*x + 10*x^2 - 6*x^3) + O(x^30)) \\ Colin Barker, Oct 31 2017
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CROSSREFS
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Sequence in context: A150242 A150243 A200316 * A121686 A245904 A128723
Adjacent sequences: A164867 A164868 A164869 * A164871 A164872 A164873
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KEYWORD
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nonn,easy
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AUTHOR
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Vincent Vatter, Aug 29 2009
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STATUS
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approved
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