OFFSET
1,2
COMMENTS
The number of free polyominoes of size 2n that have 180-degree rotational symmetry about a point that coincides with the midpoint of a side a square, independently of any reflective symmetry. Note that for polyominoes which have a hole in the center, the center of rotation will be the midpoint of a side of a square within the hole, rather than being the midpoint of a side of a square of the polyomino itself. The sequence is defined for 2n rather than n as odd-sized polyominoes cannot have the required symmetry.
CROSSREFS
KEYWORD
nonn,hard
AUTHOR
John Mason, Dec 18 2013
EXTENSIONS
More terms from John Mason, Dec 17 2021
More terms from John Mason, Apr 15 2023
STATUS
approved