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Search: id:A076907
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| A076907 |
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Meandric numbers for a river crossing two perpendicular roads at n points, beginning in the (-,-) quadrant and ending in any quadrant. |
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+0
7
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OFFSET
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0,1
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COMMENT
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a(n) = number of ways that a directed curve (or arrow) can start in the (-,-) quadrant, cross the x and y axes at exactly n points and end in any quadrant.
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FORMULA
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a(2n+1) = 2*A076906(2n+1).
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CROSSREFS
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Cf. A005316, A076876, A076906, A076875 (undirected case).
Cf. A077551 (cross x axis first).
Sequence in context: A014113 A078008 A151575 this_sequence A153897 A103774 A036052
Adjacent sequences: A076904 A076905 A076906 this_sequence A076908 A076909 A076910
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KEYWORD
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nonn,more
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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 26 2002
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EXTENSIONS
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a(6) and a(7) corrected Aug 25 2003
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