

A234010


Free polyominoes with 2n squares, having 180degree rotational symmetry about a square midside.


4



1, 2, 6, 19, 67, 241, 901, 3398, 12991, 49958, 193317, 751080, 2928956, 11455059, 44916219, 176506797, 694970938, 2741058805, 10827790934, 42831461499, 169641003412, 672657627655, 2669991663529, 10608177653227, 42184582641002
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OFFSET

1,2


COMMENTS

The number of free polyominoes of size 2n that have 180degree 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 oddsized polyominoes cannot have the required symmetry.


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KEYWORD

nonn,hard


AUTHOR



EXTENSIONS



STATUS

approved



