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 A101364 In the interior of a regular n-gon 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 3,10 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 Graeme McRae, Feb 23 2008, Table of n, a(n) for n = 3..210 EXAMPLE a(18)=54 because inside a regular 18-gon 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 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. 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. Cf. A137939: number of 5-way intersections in the interior of a regular 6n-gon. Sequence in context: A063863 A236238 A187795 * A216809 A204274 A271437 Adjacent sequences:  A101361 A101362 A101363 * A101365 A101366 A101367 KEYWORD nonn AUTHOR Graeme McRae, Dec 26 2004, revised Feb 23 2008 STATUS approved

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