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A337700
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.
4
12, 16, 20, 24, 63, 72, 315, 350, 385, 408, 767, 826, 2085, 2224, 2363, 2340, 3762, 3960, 7644, 8008, 8372, 8448, 11850, 12324, 20466, 21224, 21982, 21480, 28985, 29920, 45177, 46546, 47915, 48456, 60273, 61902, 87555, 89800, 92045, 91896, 111972, 114576
OFFSET
3,1
COMMENTS
Because of symmetry, a(n) is divisible by n.
LINKS
Lars Blomberg, Illustration for n = 3
Lars Blomberg, Illustration for n = 4
Lars Blomberg, Illustration for n = 5
Lars Blomberg, Illustration for n = 7
FORMULA
a(n) = A337702(n) - A337701(n) by Euler's formula, there being 1 hole.
CROSSREFS
KEYWORD
nonn
AUTHOR
Lars Blomberg, Sep 16 2020
STATUS
approved