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 A034185 #12 Dec 15 2021 00:13:54 %S A034185 1,3,11,21,61,111,309,565,1563,2859,7907,14459,39991,73117,202227, %T A034185 369729,1022587,1869581,5170829,9453757,26146909,47804041,132214959, %U A034185 241726811,668560673,1222320359,3380656559,6180808253,17094691953,31253992007,86441342981 %N A034185 Number of symmetric n X 3 crossword puzzle grids. %C A034185 Rules: all white squares are edge connected; at least 1 white square on every edge of grid; symmetric ones have 180-degree rotational symmetry. %H A034185 Sean A. Irvine, <a href="https://github.com/archmageirvine/joeis/blob/master/src/irvine/oeis/a034/A034185.java">Java program</a> (github) %Y A034185 Cf. A001333, A034184, A034186, A034187. %K A034185 nonn %O A034185 1,2 %A A034185 _Erich Friedman_ %E A034185 a(23)-a(31) from _Sean A. Irvine_, Aug 07 2020