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 A234009 #20 Dec 15 2021 00:10:44 %S A234009 1,1,4,10,35,114,403,1432,5239,19271,71820,269417,1017920,3864879 %N A234009 Free polyominoes with 4n squares, having 90-degree rotational symmetry about a square corner. %C A234009 The number of free polyominoes of size 4n that have 90-degree rotational symmetry about a point that coincides with the corner of a square, independently of any other symmetries. Note that for polyominoes which have a hole in the center, the center of rotation will be the corner of a square within the hole, rather than being the corner of a square of the polyomino itself. The sequence is defined for 4n rather than n as polyominoes of size not multiple of 4 cannot have the required symmetry. %Y A234009 Cf. A000105, A001933, A234006, A234007, A234008, A234010. %K A234009 nonn,hard,more %O A234009 1,3 %A A234009 _John Mason_, Dec 18 2013 %E A234009 a(8)-a(14) from _John Mason_, Dec 13 2021