%I #10 Jul 30 2012 00:44:18
%S 1,2,4,10,22,62,176,436
%N Meandric numbers for a curve crossing two perpendicular lines at n points.
%C 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 Jon Wild, <a href="/A076875/a076875.gif">Illustration of a(4) = 22</a> (ignore the arrowheads)
%F For n odd a(n) = 2*A076906(n).
%e 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 Cf. A005316, A076876, A076906, A076907 (directed case).
%K nonn,nice
%O 0,2
%A _N. J. A. Sloane_ and _Jon Wild_, Nov 24 2002
%E a(6) and a(7) corrected Aug 23 2003
%E a(7) corrected by _Robert Price_, Jul 29 2012
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