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 A343562 #43 Apr 16 2023 09:21:59 %S A343562 1,1,1,3,4,8,12,28,44,98,157,359,583,1324,2183,4961,8263,18729,31477, %T A343562 71231,120605,272380,464188,1046516,1793505,4036645,6952368,15623258, %U A343562 27026985,60645746,105327495,236021747,411377265,920654994,1609862767,3598552287 %N A343562 The number of n-cell polyominoes with a line of symmetry parallel to the edges. %C A343562 The polyominoes counted here may have additional symmetries (mirror axes along diagonals or rotational symmetries or two orthogonal mirror axes...). The mirror axes may run through cell edges or through cell centers. %H A343562 John Mason, <a href="/A343562/b343562.txt">Table of n, a(n) for n = 1..50</a> %F A343562 a(n) = A056877(n) + A142886(n) + A006746(n). %e A343562 For n=4 the 3 polyominoes are (i) the 2x2 block, (ii) the straight 1x4 block and (iii) the T. %Y A343562 Cf. A056877, A142886, A006746. %K A343562 nonn,hard %O A343562 1,4 %A A343562 _R. J. Mathar_, Sep 29 2021 %E A343562 More terms from _John Mason_, Dec 16 2021