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%I #12 Sep 16 2020 12:50:50
%S 12,16,20,24,63,72,315,350,385,408,767,826,2085,2224,2363,2340,3762,
%T 3960,7644,8008,8372,8448,11850,12324,20466,21224,21982,21480,28985,
%U 29920,45177,46546,47915,48456,60273,61902,87555,89800,92045,91896,111972,114576
%N Place two n-gons with radii 1 and 2 concentrically, forming an annular area between them. Connect all the vertices with line segments that lie entirely within that area. Then a(n) is the number of regions in that figure.
%C Because of symmetry, a(n) is divisible by n.
%H Lars Blomberg, <a href="/A337700/b337700.txt">Table of n, a(n) for n = 3..102</a>
%H Lars Blomberg, <a href="/A337700/a337700.png">Illustration for n = 3</a>
%H Lars Blomberg, <a href="/A337700/a337700_1.png">Illustration for n = 4</a>
%H Lars Blomberg, <a href="/A337700/a337700_2.png">Illustration for n = 5</a>
%H Lars Blomberg, <a href="/A337700/a337700_3.png">Illustration for n = 7</a>
%H Lars Blomberg, <a href="/A337700/a337700_4.png">Illustration for n = 10</a>
%H Lars Blomberg, <a href="/A337700/a337700_5.png">Illustration for n = 29</a>
%H Lars Blomberg, <a href="/A337700/a337700_6.png">Illustration for n = 32</a>
%F a(n) = A337702(n) - A337701(n) by Euler's formula, there being 1 hole.
%Y Cf. A337701, A337702, A337703.
%K nonn
%O 3,1
%A _Lars Blomberg_, Sep 16 2020
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