%I #20 Dec 15 2021 00:10:44
%S 1,1,4,10,35,114,403,1432,5239,19271,71820,269417,1017920,3864879
%N Free polyominoes with 4n squares, having 90degree rotational symmetry about a square corner.
%C The number of free polyominoes of size 4n that have 90degree 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 Cf. A000105, A001933, A234006, A234007, A234008, A234010.
%K nonn,hard,more
%O 1,3
%A _John Mason_, Dec 18 2013
%E a(8)a(14) from _John Mason_, Dec 13 2021
