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 A294151 #9 Dec 13 2023 04:03:52 %S A294151 1,1,2,3,8,13,36,76,202,482,1281,3277,8749,23083,62056,166498,450576, %T A294151 1220542,3321870,9057388,24769906 %N A294151 Number of perforated polyominoes with n cells, identifying mirror images. %C A294151 A perforated polyomino is a polyomino in which every cell has at least one even coordinate. %e A294151 a(4)=3 because there are 5 tetrominoes, and the skew and square tetrominoes do not lie in the perforated grid, which leaves 3. %Y A294151 Cf. A000105, number of polyominoes with n cells; A292065, number of Besźel Polycubes with n cells, distinguishing mirror images; A292157, same but identifying mirror images. %K A294151 nonn,more,hard %O A294151 1,3 %A A294151 _George Sicherman_, Oct 23 2017 %E A294151 a(16) - a(21) from _Joerg Arndt_, Dec 11 2023