%I #6 Nov 14 2021 07:26:06
%S 0,1,0,1,1,3,2,6,4,14,8,30,18,71,40,155,89,358,198,786,445,1808,998,
%T 3994,2250,9158,5068
%N Number of achiral polyomino rings of length 2n with twofold rotational symmetry.
%H D. H. Redelmeier, <a href="http://dx.doi.org/10.1016/0012365X(81)902375">Counting polyominoes: yet another attack</a>, Discrete Math., 36 (1981), 191203.
%F a(n) = A348402(n)  A348403(n).
%e a(2)=1 because of:
%e OO
%e OO
%e a(4)=1 because of:
%e OOO
%e O.O
%e OOO
%e a(5)=1 because of:
%e OOOO
%e O..O
%e OOOO
%Y Cf. A348402 (all unoriented), A348403 (chiral), A324407 (unoriented with fourfold rotational symmetry), A324408 (chiral with fourfold rotational symmetry), A324409 (achiral with fourfold rotational symmetry).
%K nonn,more
%O 1,6
%A _John Mason_, Oct 18 2021
