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 A284747 #9 Apr 02 2017 12:39:48
%S A284747 0,1,4,54,1794,99990,7955460,848584800,116816051520,20167501253760,
%T A284747 4268024125243200,1086711068022148800,327759648421871635200,
%U A284747 115567595710587359539200,47104362677165542792243200,21978200228619432098036736000,11639211300056830532862403584000,6943663015969522875618267601920000
%N A284747 Number of proper colorings of the 2n-gon with 2 instances of each of n colors under dihedral (rotational and reflectional) symmetry.
%H A284747 Omar Sehlouli, Marko Riedel, <a href="http://math.stackexchange.com/questions/2209954/">Hexagon coloring</a>
%F A284747 For n>=2, (1/4)(n-1)! + (1/4)n! + (1/(4n)) * Sum_{p=0..n} C(n,p) ((-1)^p/2^(n-p)) ((2n-p)! + p(2n-p-1)!).
%e A284747 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 A284747 Cf. A274634, A284664.
%K A284747 nonn
%O A284747 1,3
%A A284747 _Marko Riedel_, Apr 01 2017