OFFSET
1,6
COMMENTS
This sequence and its chiral and achiral versions correspond to Robert A. Russell's similar sequences for rings of fourfold rotational symmetry. The sequence does not count the mononimo or domino, referred to by Redelmeier as degenerate rings, as they are not in fact rings.
The sequence refers to rings with at least twofold (180-degree) rotational symmetry, and so includes those with (i) fourfold (90-degree) rotational symmetry, and (ii) all symmetries. - John Mason, Jan 19 2023
LINKS
D. H. Redelmeier, Counting polyominoes: yet another attack, Discrete Math., 36 (1981), 191-203.
EXAMPLE
a(2)=1 because of:
OO
OO
a(4)=1 because of:
OOO
O.O
OOO
a(5)=1 because of:
OOOO
O..O
OOOO
CROSSREFS
KEYWORD
nonn,more
AUTHOR
John Mason, Oct 18 2021
STATUS
approved