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
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
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