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A006746 Number of axially symmetric polyominoes with n cells.
(Formerly M1573)
26
0, 0, 0, 1, 2, 6, 9, 23, 38, 90, 147, 341, 564, 1294, 2148, 4896, 8195, 18612, 31349, 70983, 120357, 271921, 463712, 1045559, 1792582, 4034832, 6950579, 15619507, 27023509, 60638559, 105320716, 236006955, 411364068, 920626423, 1609836928 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,5

COMMENTS

Number of polyominoes with n cells and exactly one line of reflection symmetry, where that one line is parallel to the grid. - Joshua Zucker, Mar 08 2008

The line of reflective symmetry may pass through the center of a square or a vertex of a square. These subsets are enumerated by A349328 and A349329 respectively. - John Mason, Feb 17 2022

REFERENCES

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

LINKS

John Mason, Table of n, a(n) for n = 1..48

Tomás Oliveira e Silva, Enumeration of polyominoes

D. H. Redelmeier, Counting polyominoes: yet another attack, Discrete Math., 36 (1981), 191-203.

D. H. Redelmeier, Table 3 of Counting polyominoes...

FORMULA

a(n) = A349328(n) + A349329(n/2) for even n, otherwise a(n) = A349328(n). - John Mason, Feb 17 2022

CROSSREFS

Cf. A000105, A001168, A006746, A056877, A006748, A056878, A006747, A006749, A030227.

Sequences classifying polyominoes by symmetry group: A000105, A006746, A006747, A006748, A006749, A056877, A056878, A142886, A144553, A144554, A349328, A349329.

Sequence in context: A346673 A077237 A056879 * A205875 A243558 A244427

Adjacent sequences:  A006743 A006744 A006745 * A006747 A006748 A006749

KEYWORD

nonn

AUTHOR

N. J. A. Sloane

EXTENSIONS

Extended to n=28 by Tomás Oliveira e Silva

STATUS

approved

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Last modified June 29 09:21 EDT 2022. Contains 354910 sequences. (Running on oeis4.)