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 A057424 #13 Jan 16 2023 11:16:29 %S A057424 1,1,5,7,21,30,88,116,336,439,1271,1639,4713,6088,17452 %N A057424 Diagonally symmetric (about diagonal 2) 2n-celled polyominoes with 1 hole. %C A057424 The sequence refers to polyominoes with one hole and both diagonal symmetries but no other. - _John Mason_, Jan 16 2023 %H A057424 Tomás Oliveira e Silva, <a href="http://sweet.ua.pt/tos/animals.html">Enumeration of polyominoes</a> %Y A057424 Cf. A056882. %K A057424 nonn,more %O A057424 6,3 %A A057424 _N. J. A. Sloane_, Sep 05 2000 %E A057424 a(15)-a(20) from _John Mason_, Jan 16 2023