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 A348402 Number of unoriented polyomino rings of length 2n with twofold rotational symmetry. 3
 0, 1, 0, 1, 1, 3, 3, 9, 13, 35, 59, 147, 280, 669, 1347, 3142, 6545, 15110, 32057, 73625, 158056, 362280, 783800, 1795134, 3906573, 8946154, 19558340 (list; graph; refs; listen; history; text; internal format)
 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. FORMULA a(n) = A348403(n) + A348404(n). 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 Cf. A348403 (chiral), A348404 (achiral), A324407 (unoriented with fourfold rotational symmetry), A324408 (chiral with fourfold rotational symmetry), A324409 (achiral with fourfold rotational symmetry). Sequence in context: A183811 A303640 A091328 * A138383 A052436 A243790 Adjacent sequences: A348399 A348400 A348401 * A348403 A348404 A348405 KEYWORD nonn,more AUTHOR John Mason, Oct 18 2021 STATUS approved

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Last modified March 27 18:55 EDT 2023. Contains 361575 sequences. (Running on oeis4.)