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Search: id:A156831
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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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1
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| 1, 2, 6, 20, 66, 188, 466, 1022, 2098, 4032, 7342, 13090, 22726, 38824, 65286, 108902, 179762
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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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.
The first 8 terms were calculated by Edwin Clark. The first 17 terms were calculated by J K Haugland, and posted to the Usenet group sci.math.
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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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Sequence in context: A148473 A000718 A148474 this_sequence A027061 A083323 A111285
Adjacent sequences: A156828 A156829 A156830 this_sequence A156832 A156833 A156834
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KEYWORD
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more,nonn
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AUTHOR
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Leroy Quet (q1qq2qqq3qqqq(AT)yahoo.com), Feb 16 2009
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