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 A325408 #12 Jun 22 2019 00:45:35 %S A325408 1,5,35,436,11348,650798,78816939,19500572516,10050221059942, %T A325408 10777659213636406,24081651896803403442 %N A325408 Number of (2n+1) X (2n+1) black-and-white mirror-symmetric grids that are legal for crossword puzzles. %C A325408 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 A325408 Cf. A323838, A323839, A325409. %Y A325408 There are many other OEIS sequences related to crossword puzzles: search for "crossword puzzle" (without the quotes). %K A325408 nonn,more %O A325408 1,2 %A A325408 _Michael Kleber_, Apr 23 2019 %E A325408 a(11) added by _Akshay Ravikumar_, Jun 20 2019