This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A348404 #6 Nov 14 2021 07:26:06 %S A348404 0,1,0,1,1,3,2,6,4,14,8,30,18,71,40,155,89,358,198,786,445,1808,998, %T A348404 3994,2250,9158,5068 %N A348404 Number of achiral polyomino rings of length 2n with twofold rotational symmetry. %H A348404 D. H. Redelmeier, <a href="http://dx.doi.org/10.1016/0012-365X(81)90237-5">Counting polyominoes: yet another attack</a>, Discrete Math., 36 (1981), 191-203. %F A348404 a(n) = A348402(n) - A348403(n). %e A348404 a(2)=1 because of: %e A348404 OO %e A348404 OO %e A348404 a(4)=1 because of: %e A348404 OOO %e A348404 O.O %e A348404 OOO %e A348404 a(5)=1 because of: %e A348404 OOOO %e A348404 O..O %e A348404 OOOO %Y A348404 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 A348404 nonn,more %O A348404 1,6 %A A348404 _John Mason_, Oct 18 2021