Search: id:A076875
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%I A076875
%S A076875 1,2,4,10,22,62,176,478
%N A076875 Meandric numbers for a curve crossing two perpendicular lines at n points.
%C A076875 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.
%H A076875 Jon Wild, <a href="a076875.gif">Illustration of a(4) = 22</a> (ignore 
               the arrowheads)
%F A076875 For n odd a(n) = 2*A076906(n).
%e A076875 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.
%Y A076875 Cf. A005316, A076876, A076906, A076907 (directed case).
%Y A076875 Sequence in context: A121285 A030234 A148086 this_sequence A018111 A094987 
               A137681
%Y A076875 Adjacent sequences: A076872 A076873 A076874 this_sequence A076876 A076877 
               A076878
%K A076875 nonn,nice
%O A076875 0,2
%A A076875 N. J. A. Sloane (njas(AT)research.att.com) and Jon Wild (wild(AT)music.mcgill.ca), 
               Nov 24 2002
%E A076875 a(6) and a(7) corrected Aug 23 2003

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