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A006747
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Number of rotationally symmetric polyominoes with n cells.
(Formerly M3741)
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13
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1, 1, 5, 4, 18, 19, 73, 73, 278, 283, 1076, 1090, 4125, 4183, 15939, 16105, 61628, 62170, 239388, 240907, 932230, 936447, 3641945, 3651618, 14262540
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OFFSET
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4,3
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COMMENTS
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This sequence counts polyominoes with exactly the symmetry group C_2 generated by a 180 degree rotation.
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REFERENCES
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D. H. Redelmeier, Counting polyominoes: yet another attack, Discrete Math., 36 (1981), 191-203.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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Table of n, a(n) for n=4..28.
Tomas Oliveira e Silva, Enumeration of polyominoes
D. H. Redelmeier, Table 3 of Counting polyominoes...
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CROSSREFS
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Cf. A000105, A001168, A006746, A056877, A006748, A056878, A006747, A006749.
Sequences classifying polyominoes by symmetry group: A000105, A006746, A006747, A006748, A006749, A056877, A056878, A142886, A144553, A144554.
Sequence in context: A190728 A100791 A056883 * A184297 A108412 A205008
Adjacent sequences: A006744 A006745 A006746 * A006748 A006749 A006750
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KEYWORD
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nonn
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AUTHOR
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N. J. A. Sloane.
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EXTENSIONS
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Extended to n=28 by Tomas Oliveira e Silva.
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STATUS
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approved
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