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 A323839 #17 Apr 24 2019 06:04:37 %S A323839 1,12,312,31187,17438702,40575832476,404139015237875, %T A323839 17253201646564115439,3185118492387669085270268, %U A323839 2540927098290913247876202007100 %N A323839 Number of (2n+1) X (2n+1) black-and-white grids that are legal for crossword puzzles and have no all-black edges. %C A323839 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 180-degree rotation, and the set of white squares must be connected. %C A323839 Computed by _Michael Kleber_. %D A323839 Michael Kleber, Posting to Math Fun Mailing List, Feb 04 2019. %Y A323839 Cf. A323838, A325408, A325409 %Y A323839 There are many other OEIS sequences related to crossword puzzles: search for "crossword puzzle" (without the quotes). %K A323839 nonn,more %O A323839 1,2 %A A323839 _N. J. A. Sloane_, Feb 04 2019 %E A323839 a(4)-a(7) corrected by _Michael Kleber_, Feb 06 2019 %E A323839 a(8)-a(10) added by _Michael Kleber_, Apr 23 2019