

A101364


In the interior of a regular ngon with all diagonals drawn, the number of points where exactly four diagonals intersect.


5



0, 0, 0, 0, 0, 1, 0, 0, 0, 12, 0, 0, 0, 0, 0, 54, 0, 0, 0, 0, 0, 264, 0, 0, 0, 0, 0, 420, 0, 0, 0, 0, 0, 396, 0, 0, 0, 0, 0, 1134, 0, 0, 0, 0, 0, 1200, 0, 0, 0, 0, 0, 1296, 0, 0, 0, 0, 0, 3780, 0, 0, 0, 0, 0, 2310, 0, 0, 0, 0, 0, 2520, 0, 0, 0, 0, 0, 3276, 0, 0, 0, 0, 0, 3612, 0, 0, 0, 0, 0, 4050
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OFFSET

3,10


COMMENTS

When n is odd, there are no intersections in the interior of an ngon where more than 2 diagonals meet.
When n is not a multiple of 6, there are no intersections in the interior of an ngon 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 ngon where more than 5 diagonals meet except the center.
I checked the following conjecture up to n=210: "An ngon 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

_Graeme McRae_, Feb 23 2008, Table of n, a(n) for n = 3..210
Sequences formed by drawing all diagonals in regular polygon


EXAMPLE

a(18)=54 because inside a regular 18gon there are 54 points where exactly four diagonals intersect.


CROSSREFS

Cf. A006561, A007678.
Cf. A000332: C(n, 4) = number of intersection points of diagonals of convex ngon.
Cf. A006561: number of intersections of diagonals in the interior of regular ngon
Cf. A101363: number of 3way intersections in the interior of a regular 2ngon
Cf. A101365: number of 5way intersections in the interior of a regular ngon
Cf. A137938: number of 4way intersections in the interior of a regular 6ngon
Cf. A137939: number of 5way intersections in the interior of a regular 6ngon
Sequence in context: A063863 A236238 A187795 * A216809 A204274 A200512
Adjacent sequences: A101361 A101362 A101363 * A101365 A101366 A101367


KEYWORD

nonn


AUTHOR

Graeme McRae, Dec 26 2004, revised Feb 23 2008


STATUS

approved



