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 A341219 #19 Mar 27 2021 01:22:34 %S A341219 1,1,1,0,1,0,1,0,3,1,6,2,25,18,88 %N A341219 Number of well-connected free polyominoes with n cells, where a polyomino is defined as well-connected if, for all the squares it contains, the removal of that single square would result in an intact polyomino. %C A341219 Examples of well-connected polyominoes are rectangles and loops: %C A341219 . %C A341219 XXXX %C A341219 XXXX (rectangle) %C A341219 . %C A341219 XXX %C A341219 X X (loop) %C A341219 XXX %H A341219 John Mason, <a href="/A341219/a341219.pdf">Well connected polyominoes</a> %e A341219 a(4) = 1 because the 2 X 2 square polyomino is the only tetromino to be well-connected. The removal of any single square always results in an intact tromino. In each of the other 4 tetrominoes, there is a square whose removal would result in something that is not an intact polyomino. %Y A341219 Cf. A000105. %K A341219 nonn,more %O A341219 0,9 %A A341219 _John Mason_, Feb 07 2021