This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A384701 #19 Jun 15 2025 14:37:11 %S A384701 2,4,18,172,978,3672,11034,27241,60804,122741,232138,412263,697058 %N A384701 On a 2 X n grid of vertices, draw a circle through every unordered triple of non-collinear vertices: a(n) is the number of distinct points where circles intersect. %H A384701 Scott R. Shannon, <a href="/A384701/a384701.jpg">Image for a(2) = 4</a>. %H A384701 Scott R. Shannon, <a href="/A384701/a384701_1.jpg">Image for a(3) = 18</a>. %H A384701 Scott R. Shannon, <a href="/A384701/a384701_2.jpg">Image for a(4) = 172</a>. %H A384701 Scott R. Shannon, <a href="/A384701/a384701_3.jpg">Image for a(5) = 978</a>. %F A384701 a(n) = A384703(n) - A384702(n) + 1 by Euler's formula, for n > 1. %Y A384701 Cf. A384700 (circles), A384702 (regions), A384703 (edges), A359569, A374825, A374338, A373106. %K A384701 nonn,more %O A384701 1,1 %A A384701 _Scott R. Shannon_ and _N. J. A. Sloane_, Jun 07 2025