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 A156831 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. 2
 1, 2, 6, 20, 66, 188, 466, 1022, 2098, 4032, 7342, 13090, 22726, 38824, 65286, 108902, 179762 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 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 W. Edwin Clark. The first 17 terms were calculated by Jan Kristian Haugland, and posted to the Usenet group sci.math. LINKS 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. CROSSREFS Sequence in context: A148473 A000718 A148474 * A027061 A279460 A083323 Adjacent sequences:  A156828 A156829 A156830 * A156832 A156833 A156834 KEYWORD more,nonn AUTHOR Leroy Quet, Feb 16 2009 STATUS approved

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Last modified August 22 11:34 EDT 2019. Contains 326176 sequences. (Running on oeis4.)