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Search: id:A076875
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| A076875 |
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Meandric numbers for a curve crossing two perpendicular lines at n points. |
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+0
6
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OFFSET
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0,2
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COMMENT
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a(n) = number of ways that a curve can start in the (-,-) quadrant, cross the x and y axes at exactly n points and end in any quadrant. Line is undirected.
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LINKS
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Jon Wild, Illustration of a(4) = 22 (ignore the arrowheads)
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FORMULA
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For n odd a(n) = 2*A076906(n).
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EXAMPLE
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See illustration for a(4)=22: each of the 12 solutions shown crosses the x-axis first and ten of them are related by mirror symmetry to a corresponding curve that crosses the y-axis first, making the total a(4)=22.
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CROSSREFS
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Cf. A005316, A076876, A076906, A076907 (directed case).
Sequence in context: A121285 A030234 A148086 this_sequence A018111 A094987 A137681
Adjacent sequences: A076872 A076873 A076874 this_sequence A076876 A076877 A076878
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KEYWORD
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nonn,nice
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AUTHOR
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N. J. A. Sloane (njas(AT)research.att.com) and Jon Wild (wild(AT)music.mcgill.ca), Nov 24 2002
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EXTENSIONS
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a(6) and a(7) corrected Aug 23 2003
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