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 A239231 #38 Mar 22 2024 18:49:15 %S A239231 0,1,1,4,5,9,12,17,21,27,33,41,48,56,65,75,85,96,108,121,133,146,161, %T A239231 176,190,208 %N A239231 Heyawake numbers: maximum number of painted cells in an n X n grid, such that no two painted cells are orthogonally adjacent and the unpainted cells form a contiguous area. %C A239231 Inspired by the Japanese puzzle of the same name. %H A239231 Wikipedia, <a href="http://en.wikipedia.org/wiki/Heyawake">Heyawake</a>. %F A239231 a(n) = A239072(n-4) + 2*n - 2 for n > 4. %e A239231 If n=6, the painted cells could be A1, A3, A6, B5, C1, C3, D4, D6, E2, F1, F4, F6 (12 cells in all). %Y A239231 Cf. A239072 (makes up the inner n-4 X n-4 square of the grid). %K A239231 nonn,more %O A239231 0,4 %A A239231 _Elliott Line_, Mar 13 2014 %E A239231 Some values corrected, incorrect values removed by _Elliott Line_, Aug 21 2014 %E A239231 a(16) and a(20) corrected by _Elliott Line_ at the suggestion of Greg Malen, Sep 02 2020