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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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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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MATHEMATICA
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LinearRecurrence[{6, -10, 6}, {1, 2, 6}, 30] (* Harvey P. Dale, Oct 12 2023 *)
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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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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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