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 A351615 #17 Apr 15 2023 05:09:24 %S A351615 0,0,0,0,1,0,4,0,19,1,73,4,283,25,1090,106,4183,463,16105,1892,62170, %T A351615 7752,240907,31212,936447,125609,3651618,503165,14277519,2014826, %U A351615 55961118,8058790,219813564,32231897,865091976,128897247,3410498446,515554767,13466262587,2062445063,53245595410,8252868076,210800433323 %N A351615 Number of free polyominoes with n squares, having 180-degree rotational symmetry about the center of a square, and no other symmetry. %C A351615 The sequence enumerates a subset of the polyominoes enumerated by A006747. %H A351615 John Mason, <a href="/A351615/b351615.txt">Table of n, a(n) for n = 1..50</a> %e A351615 a(5)=1 because of: %e A351615 OO %e A351615 O %e A351615 OO %Y A351615 Cf. A006747, A234008, A351616. %K A351615 nonn %O A351615 1,7 %A A351615 _John Mason_, Feb 15 2022 %E A351615 More terms from _John Mason_, Apr 15 2023