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 A325409 #11 Jun 22 2019 00:45:43 %S A325409 1,2,20,269,8116,519400,67566361,17518353991,9334778743018, %T A325409 10240638005472594,23244895727823155064 %N A325409 Number of (2n+1) X (2n+1) black-and-white mirror-symmetric grids that are legal for crossword puzzles and have no all-black edges. %C A325409 Rules: Every white square must be "checked" (in both a horizontal and vertical word), words must be at least 3 letters long, the whole grid must be symmetric under both left-right and top-bottom mirror symmetry, and the set of white squares must be connected. %Y A325409 Cf. A323838, A323839, A325408. %Y A325409 There are many other OEIS sequences related to crossword puzzles: search for "crossword puzzle" (without the quotes). %K A325409 nonn,more %O A325409 1,2 %A A325409 _Michael Kleber_, Apr 23 2019 %E A325409 a(11) added by _Akshay Ravikumar_, Jun 20 2019