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A006747 Number of rotationally symmetric polyominoes with n cells.
(Formerly M3741)
20
0, 0, 0, 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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,6

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), 191-203.

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=1..28.

Tomás 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 Tomás Oliveira e Silva

a(1)-a(3) prepended by Andrew Howroyd, Dec 04 2018

STATUS

approved

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Last modified January 20 18:46 EST 2019. Contains 319335 sequences. (Running on oeis4.)