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A228396
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The number of permutations of length n sortable by 2 reversals.
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1
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1, 2, 6, 22, 63, 145, 288, 516, 857, 1343, 2010, 2898, 4051, 5517, 7348, 9600, 12333, 15611, 19502, 24078, 29415, 35593, 42696, 50812, 60033, 70455, 82178, 95306, 109947, 126213, 144220, 164088, 185941, 209907, 236118, 264710, 295823, 329601, 366192
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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.: -1 -(x^7 - x^6 - 3*x^5 + 7*x^4 - 4*x^3 + 7*x^2 - 4*x + 1)/(x - 1)^5.
a(n) = 8 + n*(n^3 -2*n^2 +2*n -19)/6 for n>2, a(1)=1, a(2)=2. [Bruno Berselli, Aug 22 2013]
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EXAMPLE
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There are 2 permutations of length 4 which cannot be sorted by 2 reversals.
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MATHEMATICA
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CoefficientList[Series[(1/x) (-1 - (x^7 - x^6 - 3 x^5 + 7 x^4 - 4 x^3 + 7 x^2 - 4 x + 1)/(x - 1)^5), {x, 0, 40}], x] (* Bruno Berselli, Aug 22 2013 *)
LinearRecurrence[{5, -10, 10, -5, 1}, {1, 2, 6, 22, 63, 145, 288}, 40] (* Harvey P. Dale, Mar 08 2019 *)
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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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