A076875 Meandric numbers for a curve crossing two perpendicular lines at n points.

1, 2, 4, 10, 22, 62, 176, 436

Offset: 0

Author

N. J. A. Sloane and Jon Wild, Nov 24 2002

Keywords

• nonn
• nice

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.

Examples

			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.
		

Links

• Jon Wild, Illustration of a(4) = 22 (ignore the arrowheads)

Crossrefs

Cf. A005316, A076876, A076906, A076907 (directed case).

Formula

For n odd a(n) = 2*A076906(n).

Extensions

a(6) and a(7) corrected Aug 23 2003

a(7) corrected by Robert Price, Jul 29 2012