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 A217595 #45 Dec 09 2019 05:30:30 %S A217595 1,0,1,1,2,4,7,12,35,55,144,335,710,1791,4195 %N A217595 Number of free n-celled polyominoes without holes and odd side lengths. %C A217595 If you checkerboard color the vertices of the square lattice, a(n) is the number of orthogonal polygons of area n, which alternates between white and black vertices. %H A217595 Giovanni Resta, <a href="/A217595/a217595.png">Illustration of a(10) = 55.</a> %e A217595 For n=5 the a(5)=2 pentominoes are 'I' and 'X'. %K A217595 nonn,hard,more %O A217595 1,5 %A A217595 _Thomas Dybdahl Ahle_, May 28 2013 %E A217595 a(10) from _Giovanni Resta_, Jun 06 2013 %E A217595 a(11)-a(12) from _Thomas Dybdahl Ahle_, Nov 13 2019 %E A217595 a(13)-a(15) from _Aurélien Ooms_, Nov 14-19 2019