OFFSET
2,3
COMMENTS
Same as (total number of triangles visible in convex 2n-gon with all diagonals drawn in general position) - (total number of triangles visible in regular 2n-gon with all diagonals drawn).
Number of triple-crossings of diagonals in the regular 2n+1-gon is 0.
See Sillke 1998 (where a(n) is called "T(2n)") for explanations and extensive annotated references.
LINKS
T. Sillke, Number of triangles for a convex n-gon, 1998.
EXAMPLE
With only 2 diagonals in a 4-gon, there can be no triple-crossings, so a(2) = 0.
CROSSREFS
KEYWORD
nonn
AUTHOR
Jonathan Sondow, Jul 25 2015
STATUS
approved