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 A234007 #30 Feb 02 2022 15:10:15 %S A234007 0,1,2,9,30,110,387,1419,5185,19225,71634,269250,1017260,3864267, %T A234007 14742260,56470053,217052829,836878982 %N A234007 Free polyominoes with 4n squares, having 90-degree rotational symmetry about a square corner, but not having reflective symmetry. %C A234007 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, and that have not at the same time reflective symmetry. 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 a multiple of 4 cannot have the required symmetry. %C A234007 The sequence enumerates a subset of the polyominoes enumerated by A144553. %Y A234007 Cf. A000105, A001933, A234006, A234008, A234009, A234010, A144553. %K A234007 nonn,more %O A234007 1,3 %A A234007 _John Mason_, Dec 18 2013 %E A234007 a(8)-a(13) from _Sean A. Irvine_, Jul 04 2019 %E A234007 a(14)-a(18) from _John Mason_, Feb 02 2022