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%I #5 Mar 30 2012 18:34:46
%S 0,12,54,264,420,396,1134,1200,1296,3780,2310,2520,3276,3612,4050,
%T 5088,5712,5724,7182,11400,9072,9372,10626,11088,12600,13260,14094,
%U 15960,17052,23220,19530,20928,21384,23052,26250
%N Number of 4-way intersections in the interior of a regular 6n-gon.
%C When n is odd, there are no intersections in the interior of an n-gon where more than 2 diagonals meet.
%C 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.
%C 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.
%C 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)."
%H <a href="/index/Pol#Poonen">Sequences formed by drawing all diagonals in regular polygon</a>
%e a(3)=54 because there are 54 points in the interior of an 18-gon at which exactly four diagonals intersect.
%Y Cf. A000332: C(n, 4) = number of intersection points of diagonals of convex n-gon.
%Y Cf. A006561: number of intersections of diagonals in the interior of regular n-gon.
%Y Cf. A101363: number of 3-way intersections in the interior of a regular 2n-gon.
%Y Cf. A101364: number of 4-way intersections in the interior of a regular n-gon.
%Y Cf. A101365: number of 5-way intersections in the interior of a regular n-gon.
%Y Cf. A137939: number of 5-way intersections in the interior of a regular 6n-gon
%K nonn
%O 1,2
%A _Graeme McRae_, Feb 23 2008
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