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 A348403 #8 Nov 14 2021 07:25:55 %S A348403 0,0,0,0,0,0,1,3,9,21,51,117,262,598,1307,2987,6456,14752,31859,72839, %T A348403 157611,360472,782802,1791140,3904323,8936996,19553272 %N A348403 Number of chiral pairs of polyomino rings of length 2n with twofold rotational symmetry. %H A348403 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 A348403 a(n) = A348402(n) - A348404(n). %e A348403 a(7)=1 because of: %e A348403 OOOO %e A348403 O..OO %e A348403 OO..O %e A348403 .OOOO %Y A348403 Cf. A348402 (all unoriented), A348404 (achiral), A324407 (unoriented with fourfold rotational symmetry), A324408 (chiral with fourfold rotational symmetry), A324409 (achiral with fourfold rotational symmetry). %K A348403 nonn,more %O A348403 1,8 %A A348403 _John Mason_, Oct 18 2021