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 A212807 #19 Dec 01 2018 10:15:43 %S A212807 1,1,3,4,7,4,13,13,7,16,8,23,36,58,92,101,172,281,337,541,746 %N A212807 Number of ways to place 2n points on a 2n X 2n square array of points such that no three points are collinear and the configuration has 90-degree rotational symmetry. %C A212807 No such arrangement is possible on a 2n+1 X 2n+1 grid. %H A212807 D. B. Anderson, <a href="/A212807/a212807.jpg">Sample solutions for n = 18, 20, 22, 24, 26</a> %H A212807 David Brent Anderson <a href="https://doi.org/10.1016/0097-3165(79)90025-6">Update on the no-three-in-line problem</a>, J. Combin. Theory Ser. A 27 (1979), no. 3, 365--366. MR0555806(81g:05055). %H A212807 A. Flammenkamp, <a href="http://wwwhomes.uni-bielefeld.de/achim/no3in/table.html">Solutions of the no-three-in-line problem</a> %Y A212807 Cf. A000769. %K A212807 nonn,more,fini %O A212807 1,3 %A A212807 _N. J. A. Sloane_, May 29 2012 %E A212807 a(11)-a(21) from _Giovanni Resta_, Mar 22 2013 (data from A. Flammenkamp's website)