A076875 - OEIS

%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