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 A351616 #15 Feb 26 2023 08:42:59 %S A351616 0,0,1,2,12,43,174,657,2571,9911,38633,150437,588544,2306204,9061179, %T A351616 35665587,140648710,555514177,2197298561,8702439446,34506900216, %U A351616 136972867380,544237912916,2164377093376,8614630304437 %N A351616 Number of free polyominoes with 2n squares, having 180-degree rotational symmetry about a vertex of a square, and no other symmetry. %C A351616 The sequence enumerates a subset of the polyominoes enumerated by A006747. %e A351616 a(3)=1 because of: %e A351616 OOO %e A351616 OOO %Y A351616 Cf. A006747, A234008, A351615. %K A351616 nonn,more %O A351616 1,4 %A A351616 _John Mason_, Feb 15 2022 %E A351616 a(17)-a(25) from _John Mason_, Feb 26 2023