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 A351159 #12 Feb 05 2022 03:11:08 %S A351159 0,0,0,0,0,0,1,0,0,0,2,1,3,1,6,4,9,6,20,19,32,25,64,82,114,107,217, %T A351159 332,397,442,751,1316 %N A351159 Number of polyominoes of n cells with both diagonal symmetries, for which the 180-degree rotational symmetry has an axis that coincides with the center of a square, but without 90-degree rotational symmetry. %C A351159 This sequence enumerates a subset of the polyominoes enumerated by A056878. %F A351159 For odd n, a(n) = A056878(n); for even n, a(n) = A056878(n) - A351160(n/2). %e A351159 For a(7)=1, the heptomino with exactly fourfold symmetry and axes of symmetry parallel to the diagonals of the cells is composed of two 2 X 2 squares with one cell in common. %Y A351159 Cf. A000105, A056878, A351160. %K A351159 nonn,more %O A351159 1,11 %A A351159 _John Mason_, Feb 03 2022