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 A337502 #13 Aug 30 2020 18:02:36 %S A337502 0,1,3,6,9,15,20,27,34,43 %N A337502 Minimum number of painted cells in an n X n grid to avoid unpainted tetrominoes. %e A337502 For n = 4, painting only 5 cells would leave an unpainted tetromino, but painting the following 6 cells avoids all unpainted tetrominoes: %e A337502 . . X . %e A337502 X X . X %e A337502 . . X . %e A337502 . X . . %Y A337502 Cf. A337501. %K A337502 nonn,more %O A337502 1,3 %A A337502 _Rob Pratt_, Aug 29 2020