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A076875 Meandric numbers for a curve crossing two perpendicular lines at n points. 24
1, 2, 4, 10, 22, 62, 176, 436 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

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.

LINKS

Table of n, a(n) for n=0..7.

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

FORMULA

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

EXAMPLE

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.

CROSSREFS

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

Sequence in context: A208062 A179468 A203254 * A179490 A173185 A189890

Adjacent sequences:  A076872 A076873 A076874 * A076876 A076877 A076878

KEYWORD

nonn,nice

AUTHOR

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

EXTENSIONS

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

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

STATUS

approved

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Last modified May 23 12:45 EDT 2017. Contains 286915 sequences.