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A151597
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Number of permutations of 4 indistinguishable copies of 1..n arranged in a circle with exactly 2 adjacent element pairs in decreasing order.
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2
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0, 36, 606, 4744, 30850, 186924, 1092966, 6248976, 35154954, 195310900, 1074216814, 5859372696, 31738278546, 170898434364, 915527340150, 4882812495904, 25939941401626, 137329101557316, 724792480462974, 3814697265618600, 20027160644524194, 104904174804679756
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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.: 2*x^2*(18 + 69*x - 523*x^2 + 255*x^3 - 75*x^4) / ((1 - x)^3*(1 - 5*x)^2).
a(n) = 13*a(n-1) - 58*a(n-2) + 106*a(n-3) - 85*a(n-4) + 25*a(n-5) for n>6.
(End)
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PROG
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(Magma) [0] cat [ n*(2*5^n - 16*n) : n in [2..30]]; // Wesley Ivan Hurt, Jul 16 2020
(PARI) a(n) = if(n <= 1, 0, n*(2*5^n - 16*n)) \\ Andrew Howroyd, May 04 2020
(PARI) concat(0, Vec(2*x^2*(18 + 69*x - 523*x^2 + 255*x^3 - 75*x^4) / ((1 - x)^3*(1 - 5*x)^2) + O(x^40))) \\ Colin Barker, Jul 16 2020
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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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EXTENSIONS
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STATUS
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approved
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