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 A234010 #20 Apr 15 2023 15:06:32 %S A234010 1,2,6,19,67,241,901,3398,12991,49958,193317,751080,2928956,11455059, %T A234010 44916219,176506797,694970938,2741058805,10827790934,42831461499, %U A234010 169641003412,672657627655,2669991663529,10608177653227,42184582641002 %N A234010 Free polyominoes with 2n squares, having 180-degree rotational symmetry about a square mid-side. %C A234010 The number of free polyominoes of size 2n that have 180-degree rotational symmetry about a point that coincides with the midpoint of a side a square, independently of any reflective symmetry. Note that for polyominoes which have a hole in the center, the center of rotation will be the midpoint of a side of a square within the hole, rather than being the midpoint of a side of a square of the polyomino itself. The sequence is defined for 2n rather than n as odd-sized polyominoes cannot have the required symmetry. %F A234010 a(n) = A346799(n) + A234008(n). %Y A234010 Cf. A000105, A001933, A234006, A234007, A234008, A234009, A346799. %K A234010 nonn,hard %O A234010 1,2 %A A234010 _John Mason_, Dec 18 2013 %E A234010 More terms from _John Mason_, Dec 17 2021 %E A234010 More terms from _John Mason_, Apr 15 2023