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A189566
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Number of permutations p of 1,2,...,n satisfying |p(i+2)-p(i)|<>4 and |p(j+4)-p(j)|<>2 for all i=1..n-2, j=1..n-4.
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5
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1, 2, 6, 24, 60, 208, 1184, 7840, 51636, 410272, 3836456, 39971896, 455888312, 5717233896, 78164908748, 1153568477544
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OFFSET
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1,2
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COMMENTS
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a(n) is also the number of ways to place n nonattacking pieces rook + leaper[2,4] on an n X n chessboard.
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LINKS
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FORMULA
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Asymptotic: a(n)/n! ~ 1/e^4.
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CROSSREFS
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KEYWORD
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nonn,hard
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AUTHOR
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STATUS
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approved
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