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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, 2, 6, 20, 66, 188, 466, 1022, 2098, 4032, 7342, 13090, 22726, 38824, 65286, 108902, 179762
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| 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
| 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
| Sequence in context: A148473 A000718 A148474 * A027061 A083323 A174846
Adjacent sequences: A156828 A156829 A156830 * A156832 A156833 A156834
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KEYWORD
| more,nonn
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AUTHOR
| Leroy Quet, Feb 16 2009
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