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 A234007 Free polyominoes with 4n squares, having 90-degree rotational symmetry about a square corner, but not having reflective symmetry. 12
 0, 1, 2, 9, 30, 110, 387, 1419, 5185, 19225, 71634, 269250, 1017260, 3864267, 14742260, 56470053, 217052829, 836878982 (list; graph; refs; listen; history; text; internal format)
 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, 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. The sequence enumerates a subset of the polyominoes enumerated by A144553. LINKS Table of n, a(n) for n=1..18. CROSSREFS Cf. A000105, A001933, A234006, A234008, A234009, A234010, A144553. Sequence in context: A289901 A261330 A101604 * A203365 A181954 A343565 Adjacent sequences: A234004 A234005 A234006 * A234008 A234009 A234010 KEYWORD nonn,more AUTHOR John Mason, Dec 18 2013 EXTENSIONS a(8)-a(13) from Sean A. Irvine, Jul 04 2019 a(14)-a(18) from John Mason, Feb 02 2022 STATUS approved

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Last modified June 10 17:56 EDT 2023. Contains 363206 sequences. (Running on oeis4.)