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

0,1

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.

Links

Table of n, a(n) for n=0..20.

Formula

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

Crossrefs

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

Cf. A077551 (cross x axis first).

Sequence in context: A229733 A051765 A265987 * A260785 A153897 A103774

Adjacent sequences: A076904 A076905 A076906 * A076908 A076909 A076910

Keyword

nonn,more

Author

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

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

Status

approved