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 A076875 Meandric numbers for a curve crossing two perpendicular lines at n points. 24
 1, 2, 4, 10, 22, 62, 176, 436 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS 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. LINKS Jon Wild, Illustration of a(4) = 22 (ignore the arrowheads) FORMULA For n odd a(n) = 2*A076906(n). EXAMPLE 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. CROSSREFS Cf. A005316, A076876, A076906, A076907 (directed case). Sequence in context: A208062 A179468 A203254 * A179490 A173185 A294680 Adjacent sequences:  A076872 A076873 A076874 * A076876 A076877 A076878 KEYWORD nonn,nice AUTHOR N. J. A. Sloane and Jon Wild, Nov 24 2002 EXTENSIONS a(6) and a(7) corrected Aug 23 2003 a(7) corrected by Robert Price, Jul 29 2012 STATUS approved

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Last modified May 25 10:33 EDT 2020. Contains 334592 sequences. (Running on oeis4.)