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 A284664 #22 Apr 05 2017 04:58:10
%S A284664 0,1,5,96,3528,199620,15908400,1697149440,233631921600,40335000693120,
%T A284664 8536048230528000,2173422135804796800,655519296840629760000,
%U A284664 231135191421131129932800,94208725354330431747302400,43956400457238853734678528000,23278422600113660887881093120000
%N A284664 Number of proper colorings of the 2n-gon with 2 instances of each of n colors under rotational symmetry.
%H A284664 Omar Sehlouli, Marko Riedel, <a href="http://math.stackexchange.com/questions/2209954/">Hexagon coloring</a>
%H A284664 Marko Riedel, <a href="http://www.mathematik.uni-stuttgart.de/~riedelmo/images/noniso-circnoadj3.png">Image of the five colorings of the hexagon (n=3).</a>
%F A284664 For n>=2, (1/2)(n-1)! + (1/(2n)) * Sum_{p=0..n} C(n,p) ((-1)^p/2^(n-p)) ((2n-p)! + p(2n-p-1)!).
%e A284664 When n=2 the coloring of the nodes of the square with two instances each of black and white must alternate and a rotation by Pi/4 takes one coloring to the other, so there is just one coloring.
%Y A284664 Cf. A274634, A284747.
%K A284664 nonn
%O A284664 1,3
%A A284664 _Marko Riedel_, Mar 31 2017