OFFSET
1,3
COMMENTS
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.
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
John Mason, Dec 18 2013
EXTENSIONS
a(8)-a(14) from John Mason, Dec 13 2021
STATUS
approved