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 A057423 #15 Jan 16 2023 11:16:44 %S A057423 1,0,5,4,20,18,79,84,304,332,1149,1309,4312,5005,16136,19049,60347, %T A057423 71979,225867,271781,846667,1025315,3179798,3871607,11966976,14633947, %U A057423 45131841,55395495,170560833,210008554,645861961,797419730,2450316125,3032540599 %N A057423 Diagonally symmetric n-celled polyominoes with 1 hole. %C A057423 The sequence refers to polyominoes with one hole and precisely one diagonal symmetry. - _John Mason_, Jan 16 2023 %H A057423 Tomás Oliveira e Silva, <a href="http://sweet.ua.pt/tos/animals.html">Enumeration of polyominoes</a> %Y A057423 Cf. A056881. %K A057423 nonn %O A057423 7,3 %A A057423 _N. J. A. Sloane_, Sep 05 2000 %E A057423 a(29)-a(49) from _John Mason_, Jan 16 2023