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%I #10 Sep 16 2020 12:50:56
%S 9,12,15,18,49,56,252,280,308,312,650,700,1845,1968,2091,1962,3401,
%T 3580,7056,7392,7728,7560,11050,11492,19305,20020,20735,19320,27497,
%U 28384,43164,44472,45780,45720,57794,59356,84357,86520,88683,86730,108145,110660
%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 vertices in that figure.
%C Because of symmetry, a(n) is divisible by n.
%C See A337700 for illustrations.
%H Lars Blomberg, <a href="/A337701/b337701.txt">Table of n, a(n) for n = 3..102</a>
%F a(n) = A337702(n) - A337700(n) by Euler's formula, there being 1 hole.
%Y Cf. A337700, A337702, A337703.
%K nonn
%O 3,1
%A _Lars Blomberg_, Sep 16 2020
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