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 A057420 #9 Jun 04 2022 19:39:03 %S A057420 1,3,8,25,45,137,221,650,1006,2857,4370,12050,18417,49516,76037, %T A057420 200184,309411,800440,1245934,3176482 %N A057420 Axially symmetric n-celled polyominoes with 1 hole. %H A057420 Tomás Oliveira e Silva, <a href="http://sweet.ua.pt/tos/animals.html">Enumeration of polyominoes</a> %Y A057420 Cf. A006746, A057418. %K A057420 nonn %O A057420 9,2 %A A057420 _N. J. A. Sloane_, Sep 05 2000