A348404 - OEIS

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A348404

Number of achiral polyomino rings of length 2n with twofold rotational symmetry.

0, 1, 0, 1, 1, 3, 2, 6, 4, 14, 8, 30, 18, 71, 40, 155, 89, 358, 198, 786, 445, 1808, 998, 3994, 2250, 9158, 5068

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,6

LINKS

Table of n, a(n) for n=1..27.

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

FORMULA

a(n) = A348402(n) - A348403(n).

EXAMPLE

a(2)=1 because of:

a(4)=1 because of:

OOO

O.O

OOO

a(5)=1 because of:

OOOO

O..O

OOOO

CROSSREFS

Cf. A348402 (all unoriented), A348403 (chiral), A324407 (unoriented with fourfold rotational symmetry), A324408 (chiral with fourfold rotational symmetry), A324409 (achiral with fourfold rotational symmetry).

Sequence in context: A162255 A164073 A286595 * A218615 A177828 A248479

Adjacent sequences: A348401 A348402 A348403 * A348405 A348406 A348407

KEYWORD

nonn,more

AUTHOR

John Mason, Oct 18 2021

STATUS

approved