|
|
A076875
|
|
Meandric numbers for a curve crossing two perpendicular lines at n points.
|
|
24
|
|
|
|
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
|
|
|
FORMULA
|
|
|
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
|
|
|
KEYWORD
|
nonn,nice
|
|
AUTHOR
|
|
|
EXTENSIONS
|
a(6) and a(7) corrected Aug 23 2003
|
|
STATUS
|
approved
|
|
|
|