

A006747


Number of rotationally symmetric polyominoes with n cells.
(Formerly M3741)


13



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

4,3


COMMENTS

This sequence counts polyominoes with exactly the symmetry group C_2 generated by a 180 degree rotation.


REFERENCES

D. H. Redelmeier, Counting polyominoes: yet another attack, Discrete Math., 36 (1981), 191203.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).


LINKS

Table of n, a(n) for n=4..28.
Tomas Oliveira e Silva, Enumeration of polyominoes
D. H. Redelmeier, Table 3 of Counting polyominoes...


CROSSREFS

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


KEYWORD

nonn


AUTHOR

N. J. A. Sloane.


EXTENSIONS

Extended to n=28 by Tomas Oliveira e Silva.


STATUS

approved



