A076907 Meandric numbers for a river crossing two perpendicular roads at n points, beginning in the (-,-) quadrant and ending in any quadrant.

2, 2, 6, 10, 32, 62, 210, 436, 1540, 3346, 12192, 27344, 102054, 234388, 891574, 2085940, 8057844, 19134786, 74864648, 179968564, 711708544

Offset: 0

Author

N. J. A. Sloane and Jon Wild, Nov 26 2002

Keywords

• nonn
• more

Comments

a(n) = number of ways that a directed curve (or arrow) can start in the (-,-) quadrant, cross the x and y axes at exactly n points and end in any quadrant.

Crossrefs

Cf. A005316, A076876, A076906, A076875 (undirected case).

Cf. A077551 (cross x axis first).

Formula

a(2n+1) = 2*A076906(2n+1).

Extensions

a(6) and a(7) corrected Aug 25 2003

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

a(8)-a(20) from Robert Price, Aug 01 2012