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A156831
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Let a(n) = the number of permutations (p(1),p(2),p(3)...,p(n)) of (1,2,3,...,n) where, if each (m,p(m)) is plotted on a graph, then the entire set P of the n of these plotted points would be on the perimeter of the convex hull of P.
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2
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1, 2, 6, 20, 66, 188, 466, 1022, 2098, 4032, 7342, 13090, 22726, 38824, 65286, 108902, 179762
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OFFSET
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1,2
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COMMENTS
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Three points that are consecutive along the perimeter of the convex hull may be along the same line, in some of the permutations that are counted.
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LINKS
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EXAMPLE
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For n=5, (p(1),p(2),p(3),p(4),p(5)) = (1,3,5,2,4) would be included in the count, but (1,4,3,2,5) would not because point (3,3) is not on the perimeter of the convex hull of P.
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CROSSREFS
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KEYWORD
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more,nonn
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AUTHOR
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STATUS
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approved
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