OFFSET
1,3
COMMENTS
When n is odd, there are no intersections in the interior of an n-gon where more than 2 diagonals meet.
When n is not a multiple of 6, there are no intersections in the interior of an n-gon where more than 3 diagonals meet except the center.
When n is not a multiple of 30, there are no intersections in the interior of an n-gon where more than 5 diagonals meet except the center.
I checked the following conjecture up to n=210: "An n-gon with n=30k has 5n points where 6 or 7 diagonals meet and no interior point other than the center where more than 7 diagonals meet; If k is odd, then 6 diagonals meet in each of 4n points and 7 diagonals meet in each of n points; If k is even, then no groups of exactly 6 diagonals meet in a point, while exactly 7 diagonals meet in each of 5n points (all points interior excluding the center)."
LINKS
Seiichi Manyama, Table of n, a(n) for n = 1..10000
B. Poonen and M. Rubinstein, The number of intersection points made by the diagonals of a regular polygon, arXiv:math/9508209 [math.MG]; some typos in the published version are corrected in the revisions from 2006.
FORMULA
a(n) = A101365(6*n). - Seiichi Manyama, Jul 20 2024
EXAMPLE
a(3) = 54 because there are 54 points in the interior of an 18-gon at which exactly five diagonals meet.
CROSSREFS
Cf. A000332: C(n, 4) = number of intersection points of diagonals of convex n-gon..
Cf. A006561: number of intersections of diagonals in the interior of regular n-gon.
Cf. A101363: number of 3-way intersections in the interior of a regular 2n-gon.
Cf. A101364: number of 4-way intersections in the interior of a regular n-gon.
Cf. A101365: number of 5-way intersections in the interior of a regular n-gon.
Cf. A137938: number of 4-way intersections in the interior of a regular 6n-gon.
KEYWORD
nonn
AUTHOR
Graeme McRae, Feb 23 2008
EXTENSIONS
More terms from Seiichi Manyama, Jul 20 2024
STATUS
approved