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 A323838 #18 Apr 24 2019 03:19:19 %S A323838 1,15,397,35324,18446134,41752489852,409764131469787, %T A323838 17368974525101032256,3195393813996265512672344, %U A323838 2544868518166378264304163732974 %N A323838 Number of (2n+1) X (2n+1) black-and-white grids that are legal for crossword puzzles. %C A323838 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 A323838 Computed by _Michael Kleber_. %D A323838 Michael Kleber, Posting to Math Fun Mailing List, Feb 04 2019. %Y A323838 Cf. A323839, A325408, A325409. %Y A323838 There are many other OEIS sequences related to crossword puzzles: search for "crossword puzzle" (without the quotes). %K A323838 nonn,more %O A323838 1,2 %A A323838 _N. J. A. Sloane_, Feb 04 2019 %E A323838 a(4)-a(7) corrected by _Michael Kleber_, Feb 06 2019 %E A323838 a(8)-a(10) added by _Michael Kleber_, Apr 23 2019